Document:

Exhibit 10.1

 

EXECUTION VERSION

 

THIS PROMISSORY NOTE (THIS “NOTE”) HAS NOT
BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “SECURITIES ACT”). THIS NOTE HAS BEEN ACQUIRED
FOR INVESTMENT ONLY AND MAY NOT BE SOLD, TRANSFERRED OR ASSIGNED IN THE ABSENCE OF REGISTRATION OF THE RESALE THEREOF UNDER THE
SECURITIES ACT OR AN OPINION OF COUNSEL REASONABLY SATISFACTORY IN FORM, SCOPE AND SUBSTANCE TO THE COMPANY THAT SUCH REGISTRATION
IS NOT REQUIRED.

 

PROMISSORY NOTE

 

	Principal Amount: Up to $300,000	Dated as of August 18, 2020

 

FirstMark Horizon Acquisition Corp., a Delaware
corporation (“Maker”), promises to pay to the order of FirstMark Horizon Sponsor LLC, a Delaware limited liability
company, or its registered assigns or successors in interest (collectively, “Payee”), or order, the principal
sum of Three Hundred Thousand Dollars ($300,000) or such lesser amount as shall have been advanced by Payee to Maker and shall
remain unpaid under this Note on the Maturity Date (as defined below) in lawful money of the United States of America, on the terms
and conditions described below. All payments on this Note shall be made by check or wire transfer of immediately available funds
or as otherwise determined by Maker to such account as Payee may from time to time designate by written notice in accordance with
the provisions of this Note.

 

1. Principal.
The entire unpaid principal balance of this Note shall be due and payable in full on the earlier of: (i) December 31, 2020,
and (ii) the date on which Maker consummates an initial public offering of its securities (such earlier date of (i) and (ii), the
“Maturity Date”), unless accelerated upon the occurrence of an Event of Default (as defined below). The principal
balance may be prepaid at any time by Maker, at its election and without penalty. Under no circumstances shall any individual,
including but not limited to any officer, director, employee or shareholder of Maker, be obligated personally for any obligations
or liabilities of Maker hereunder.

 

2. Drawdown
Requests. Maker and Payee agree that Maker may request, from time to time, up to Three Hundred Thousand Dollars ($300,000)
in draw downs under this Note to be used for costs and expenses related to Maker’s proposed initial public offering of its
securities (the “IPO”), including its formation. The principal of this Note may be drawn down from time to time
prior to the Maturity Date upon request from Maker to Payee (each, a “Drawdown Request”). Each Drawdown Request
must state the amount to be drawn down, and must not be an amount less than Ten Thousand Dollars ($10,000) unless agreed upon by
Maker and Payee. Payee shall fund each Drawdown Request no later than five (5) business days after receipt of a Drawdown Request;
provided, however, that the maximum amount of drawdowns outstanding under this Note at any time may not exceed Three
Hundred Thousand Dollars ($300,000). No fees, payments or other amounts shall be due to Payee in connection with, or as a result
of, any Drawdown Request by Maker.

 

3. Interest.
No interest shall accrue on the unpaid principal balance of this Note.

 

4. Application
of Payments. All payments shall be applied first to payment in full of any costs incurred in the collection of any sum due
under this Note, including (without limitation) reasonable attorney’s fees, then to the payment in full of any late charges
and finally to the reduction of the unpaid principal balance of this Note.

 

5. Events
of Default. The following shall constitute an event of default (“Event of Default”):

 

     

    

    

 

(a) Failure
to Make Required Payments. Failure by Maker to pay the principal amount due pursuant to this Note on the Maturity Date.

 

(b) Voluntary
Bankruptcy, Etc. The commencement by Maker of a voluntary case under any applicable bankruptcy, insolvency, reorganization,
rehabilitation or other similar law, or the consent by it to the appointment of or taking possession by a receiver, liquidator,
assignee, trustee, custodian, sequestrator (or other similar official) of Maker or for any substantial part of its property, or
the making by it of any assignment for the benefit of creditors, or the failure of Maker generally to pay its debts as such debts
become due, or the taking of corporate action by Maker in furtherance of any of the foregoing.

 

(c) Involuntary
Bankruptcy, Etc. The entry of a decree or order for relief by a court having jurisdiction in the premises in respect of Maker
in an involuntary case under any applicable bankruptcy, insolvency or other similar law, or appointing a receiver, liquidator,
assignee, custodian, trustee, sequestrator (or similar official) of Maker or for any substantial part of its property, or ordering
the winding-up or liquidation of its affairs, and the continuance of any such decree or order unstayed and in effect for a period
of sixty (60) consecutive days.

 

6. Remedies.

 

(a) Upon
the occurrence of an Event of Default specified in Section 5(a) hereof, Payee may, by written notice to Maker, declare this Note
to be due immediately and payable, whereupon the unpaid principal amount of this Note, and all other amounts payable thereunder,
shall become immediately due and payable without presentment, demand, protest or other notice of any kind, all of which are hereby
expressly waived, anything contained herein or in the documents evidencing the same to the contrary notwithstanding.

 

(b) Upon
the occurrence of an Event of Default specified in Sections 5(b) or 5(c), the unpaid principal balance of this Note, and all other
sums payable with regard to this Note, shall automatically and immediately become due and payable, in all cases without any action
on the part of Payee.

 

7. Waivers.
Maker and all endorsers and guarantors of, and sureties for, this Note waive presentment for payment, demand, notice of dishonor,
protest, and notice of protest with regard to the Note, all errors, defects and imperfections in any proceedings instituted by
Payee under the terms of this Note, and all benefits that might accrue to Maker by virtue of any present or future laws exempting
any property, real or personal, or any part of the proceeds arising from any sale of any such property, from attachment, levy or
sale under execution, or providing for any stay of execution, exemption from civil process, or extension of time for payment; and
Maker agrees that any real estate that may be levied upon pursuant to a judgment obtained by virtue hereof, on any writ of execution
issued hereon, may be sold upon any such writ in whole or in part in any order desired by Payee.

 

8. Unconditional
Liability. Maker hereby waives all notices in connection with the delivery, acceptance, performance, default, or enforcement
of the payment of this Note, and agrees that its liability shall be unconditional, without regard to the liability of any other
party, and shall not be affected in any manner by any indulgence, extension of time, renewal, waiver or modification granted or
consented to by Payee, and consents to any and all extensions of time, renewals, waivers, or modifications that may be granted
by Payee with respect to the payment or other provisions of this Note, and agrees that additional makers, endorsers, guarantors,
or sureties may become parties hereto without notice to Maker or affecting Maker’s liability hereunder.

 

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9. Notices.
All notices, statements or other documents which are required or contemplated by this Note shall be: (i) in writing and delivered
personally or sent by first class registered or certified mail, overnight courier service or facsimile or electronic transmission
to the address designated in writing, (ii) by facsimile to the number most recently provided to such party or such other address
or fax number as may be designated in writing by such party or (iii) by electronic mail, to the electronic mail address most
recently provided to such party or such other electronic mail address as may be designated in writing by such party. Any notice
or other communication so transmitted shall be deemed to have been given on the day of delivery, if delivered personally, on the
business day following receipt of written confirmation, if sent by facsimile or electronic transmission, one (1) business day after
delivery to an overnight courier service or five (5) days after mailing if sent by mail.

 

10. Construction.
THIS NOTE SHALL BE CONSTRUED AND ENFORCED IN ACCORDANCE WITH THE LAWS OF NEW YORK.

 

11. Severability.
Any provision contained in this Note which is prohibited or unenforceable in any jurisdiction shall, as to such jurisdiction,
be ineffective to the extent of such prohibition or unenforceability without invalidating the remaining provisions hereof, and
any such prohibition or unenforceability in any jurisdiction shall not invalidate or render unenforceable such provision in any
other jurisdiction.

 

12. Trust
Waiver. Notwithstanding anything herein to the contrary, Payee hereby waives any and all right, title, interest or claim of
any kind (“Claim”) in or to any distribution of or from the trust account to be established in which proceeds
of the IPO (including the deferred underwriting discounts and commissions) and proceeds of the sale of the warrants issued in a
private placement to occur in connection with the consummation of the IPO are to be deposited, as described in greater detail in
the registration statement and prospectus to be filed with the Securities and Exchange Commission in connection with the IPO, and
hereby agrees not to seek recourse, reimbursement, payment or satisfaction for any Claim against the trust account for any reason
whatsoever.

 

13. Amendment;
Waiver. Any amendment hereto or waiver of any provision hereof may be made with, and only with, the written consent of Maker
and Payee.

 

14. Assignment.
No assignment or transfer of this Note or any rights or obligations hereunder may be made by any party hereto (by operation
of law or otherwise) without the prior written consent of the other party hereto and any attempted assignment without the required
consent shall be void.

 

[Signature page follows]

 

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IN WITNESS WHEREOF, Maker, intending
to be legally bound hereby, has caused this Note to be duly executed by the undersigned as of the day and year first above written.

 

	 	FIRSTMARK
    HORIZON ACQUISITION CORP.
	 	 
	 	By:	/s/
    Eric Cheung
	 	 	Name: 	Eric
    Cheung
	 	 	Title:	Secretary

 

	Agreed
    and Acknowledged:	 
	 	 
	FIRSTMARK
    HORIZON SPONSOR LLC	 
	 	 
	By:
    FirstMark Capital LLC, as sole member	 
	 	 
	By:	/s/
    Eric Cheung	 
	 	Name: 	Eric
    Cheung	 
	 	Title:	General
    Counsel	 

 

[Signature Page to Promissory Note]Exhibit 10.2

 

[●], 2020

 

FirstMark Horizon Acquisition Corp.

100 5th Ave, 3rd Floor

New York, NY 10011

 

		Re:	Initial Public Offering

 

Ladies and Gentlemen:

 

This letter (this “Letter Agreement”)
is being delivered to you in accordance with the Underwriting Agreement (the “Underwriting Agreement”)
entered into or proposed to be entered into by and between FirstMark Horizon Acquisition Corp., a Delaware corporation (the “Company”),
and Credit Suisse Securities (USA) LLC, as the representative (“Representative”) of the several underwriters
named therein (each an “Underwriter” and collectively, the “Underwriters”),
relating to an underwritten initial public offering (the “Public Offering”), of up to 34,500,000 of the
Company’s units (including up to 4,500,000 units that may be purchased to cover the Underwriters’ option to purchase
additional units, if any) (the “Units”), each comprised of one share of Class A common stock of
the Company, par value $0.0001 per share (“Class A Common Stock”), and one-third of one redeemable
warrant (each whole warrant, a “Warrant”). Each Warrant entitles the holder thereof to purchase one share
of Class A Common Stock at a price of $11.50 per share, subject to adjustment. The Units shall be sold in the Public Offering
pursuant to a registration statement on Form S-1 and prospectus (the “Prospectus”) filed by the Company
with the Securities and Exchange Commission (the “Commission”). Certain capitalized terms used herein
are defined in paragraph 11 hereof.

 

In order to induce the Company and the Underwriters
to enter into the Underwriting Agreement and to proceed with the Public Offering and for other good and valuable consideration,
the receipt and sufficiency of which are hereby acknowledged, FirstMark Horizon Sponsor LLC, a Delaware limited liability company
(the “Sponsor”), and the other undersigned persons (each such other undersigned persons, an “Insider”
and collectively, the “Insiders”), each hereby agrees, severally but not jointly, with the Company as
follows:

 

1. The
Sponsor and each Insider agrees that if the Company seeks stockholder approval of a proposed Business Combination, then in connection
with such proposed Business Combination, it, he or she shall (i) vote any Shares owned by it, him or her in favor of any proposed
Business Combination (including any proposals recommended by the Company’s Board of Directors in connection with such Business
Combination) and (ii) not redeem any Shares owned by it, him or her in connection with such stockholder approval.

 

    	 	 	 

    	 	 

    

 

2. The
Sponsor and each Insider hereby agrees that in the event that the Company fails to consummate a Business Combination within 24
months from the closing of the Public Offering, or such later period approved by the Company’s stockholders in accordance
with the Company’s amended and restated certificate of incorporation, the Sponsor and each Insider shall take all reasonable
steps to cause the Company to (i) cease all operations except for the purpose of winding up, (ii) as promptly as reasonably possible
but not more than ten (10) business days thereafter, subject to lawfully available funds therefor, redeem 100% of the shares of
Class A Common Stock sold as part of the Units in the Public Offering (the “Offering Shares”), at a per
share price, payable in cash, equal to the aggregate amount then on deposit in the Trust Account, including interest (which interest
shall be net of taxes payable and less up to $100,000 of interest to pay dissolution expenses), divided by the number of then outstanding
Offering Shares, which redemption will completely extinguish all Public Stockholders’ rights as stockholders (including the
right to receive further liquidation distributions, if any) and (iii) as promptly as reasonably possible following such redemption,
subject to the approval of the Company’s remaining stockholders and the Company’s board of directors, dissolve and
liquidate, subject in each case to the Company’s obligations under Delaware law to provide for claims of creditors and the
other requirements of applicable law. The Sponsor and each Insider agrees to not propose any amendment to the Company’s amended
and restated certificate of incorporation (A) to modify the substance or timing of the Company’s obligation to allow redemption
in connection with the Company’s initial Business Combination or to redeem 100% of the Offering Shares if the Company does
not complete its initial Business Combination within 24 months from the closing of the Public Offering or (B) with respect to any
other provision relating to stockholders’ rights or pre-initial Business Combination activity, unless the Company provides
its Public Stockholders with the opportunity to redeem their Offering Shares upon approval of any such amendment at a per share
price, payable in cash, equal to the aggregate amount then on deposit in the Trust Account, including interest (which interest
shall be net of taxes payable), divided by the number of then outstanding Offering Shares.

 

The Sponsor and each Insider acknowledges
that it, he or she has no right, title, interest or claim of any kind in or to any monies held in the Trust Account or any other
asset of the Company as a result of any liquidation of the Company with respect to the Founder Shares held by it. The Sponsor and
each Insider hereby further waives, with respect to any Shares held by it, him or her, if any, any redemption rights it, he or
she may have in connection with (x) the consummation of a Business Combination, including, without limitation, any such rights
available in the context of a stockholder vote to approve such Business Combination or in the context of a tender offer made by
the Company to purchase shares of Class A Common Stock and (y) a stockholder vote to approve an amendment to the Company’s
amended and restated certificate of incorporation (A) to modify the substance or timing of the Company’s obligation to allow
redemptions in connection with the Company’s initial Business Combination or to redeem 100% of the Offering Shares if the
Company has not consummated its initial Business Combination within 24 months from the closing of the Public Offering or (B) with
respect to any other provision relating to stockholders’ rights or pre-initial Business Combination activity (although the
Sponsor and the Insiders shall be entitled to redemption and liquidation rights with respect to any Offering Shares it or they
hold if the Company fails to consummate a Business Combination within 24 months from the date of the closing of the Public Offering).

 

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3. Notwithstanding
the provisions set forth in paragraphs 7(a) and (b) below, during the period commencing on the effective date of the Underwriting
Agreement and ending 180 days after such date, The Sponsor and each Insider shall not, without the prior written consent of the
Representative, (i) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract
to sell, grant any option, right or warrant to purchase, or otherwise transfer or dispose of, directly or indirectly, or file with,
or submit to, the Commission a registration statement under the Securities Act of 1933, as amended (the “Securities
Act”), relating to any Units, shares of Class A Common Stock, Founder Shares, Warrants or any securities convertible
into, or exercisable, or exchangeable for, any Units, shares of Class A Common Stock, Founder Shares, or Warrants, or publicly
disclose the intention to undertake any of the foregoing, or (ii) enter into any swap or other arrangement that transfers, in whole
or in part, any of the economic consequences of ownership of any Units, shares of Class A Common Stock, Founder Shares, or Warrants
or any such other securities, whether any such transaction described in clause (i) or (ii) above is to be settled by delivery of
units or such other securities, in cash or otherwise; provided, however, that the foregoing does not apply to the
forfeiture of any Founder Shares pursuant to their terms or any transfer of Founder Shares to any current or future independent
director of the company (as long as such current or future independent director transferee is subject to this Letter Agreement
or executes an agreement substantially identical to the terms of this Letter Agreement, as applicable to directors and officers
at the time of such transfer; and as long as, to the extent any Section 16 reporting obligation is triggered as a result of such
transfer, any related Section 16 filing includes a practical explanation as to the nature of the transfer). Each of the Insiders
and the Sponsor acknowledges and agrees that, prior to the effective date of any release or waiver, of the restrictions set forth
in this paragraph 3 or paragraph 7 below, the Company may announce the impending release or waiver by press release through a major
news service at least two business days before the effective date of the release or waiver. The provisions of this paragraph will
not apply if (i) the release or waiver is effected solely to permit a transfer of securities that is not for consideration and
(ii) the transferee has agreed in writing to be bound by the same terms described in this Letter Agreement to the extent and for
the duration that such terms remain in effect at the time of the transfer.

 

4. In
the event of the liquidation of the Trust Account, the Sponsor (which for purposes of clarification shall not extend to any other
stockholders, members or managers of the Sponsor) agrees to indemnify and hold harmless the Company against any and all loss, liability,
claim, damage and expense whatsoever (including, but not limited to, any and all legal or other expenses reasonably incurred in
investigating, preparing or defending against any litigation, whether pending or threatened, or any claim whatsoever) to which
the Company may become subject as a result of any claim by (i) any third party (other than the Company’s independent
registered public accounting firm) for services rendered or products sold to the Company or (ii) a prospective target business
with which the Company has discussed entering into a transaction agreement (a “Target”); provided,
however, that such indemnification of the Company by the Sponsor shall apply only to the extent necessary to ensure that
such claims by a third party for services rendered (other than the Company’s independent registered public accounting firm)
or products sold to the Company or a Target do not reduce the amount of funds in the Trust Account to below (i) $10.00 per
Offering Share or (ii) such lesser amount per Offering Share held in the Trust Account as of the date of the liquidation of the
Trust Account due to reductions in the value of the trust assets as of the date of the liquidation of the Trust Account, in each
case, net of the amount of interest which may be withdrawn to pay taxes, except as to any claims by a third party who executed
a waiver of any and all rights to seek access to the Trust Account and except as to any claims under the Company’s indemnity
of the Underwriters against certain liabilities, including liabilities under the Securities Act of 1933, as amended. In the event
that any such executed waiver is deemed to be unenforceable against such third party, the Sponsor shall not be responsible to the
extent of any liability for such third party claims. The Sponsor shall have the right to defend against any such claim with counsel
of its choice reasonably satisfactory to the Company if, within 15 days following written receipt of notice of the claim to the
Sponsor, the Sponsor notifies the Company in writing that it shall undertake such defense.

 

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5. (a)
To the extent that the Underwriters do not exercise their option to purchase up to an additional 4,500,000 Units within 45 days
from the date of the Prospectus (and as further described in the Prospectus), the Sponsor agrees that it shall forfeit, at no cost,
a number of Founder Shares in the aggregate equal to 1,125,000 multiplied by a fraction, (i) the numerator of which is 4,500,000
minus the number of Units purchased by the Underwriters upon the exercise of their option to purchase additional Units and (ii)
the denominator of which is 4,500,000. All references in this Letter Agreement to Founder Shares of the Company being forfeited
shall take effect as a contribution of such Founder Shares to the Company’s capital as a matter of Delaware law. The forfeiture
will be adjusted to the extent that the option to purchase additional Units is not exercised in full by the Underwriters so that
the number of Founder Shares will equal an aggregate of 20.0% of the Company’s issued and outstanding Shares after the Public
Offering. The Initial Stockholders further agree that to the extent that the size of the Public Offering is increased or decreased,
the Company will effect a stock dividend or stock repurchase or redemption, as applicable, immediately prior to the consummation
of the Public Offering in such amount as to maintain the number of Founder Shares at 20.0% of the Company’s issued and outstanding
Shares upon the consummation of the Public Offering. In connection with such increase or decrease in the size of the Public Offering,
then (A) the references to 4,500,000 in the numerator and denominator of the formula in the first sentence of this paragraph
shall be changed to a number equal to 15.0% of the number of shares of Class A Common Stock included in the Units issued in the
Public Offering, (B) the reference to 1,125,000 in the formula set forth in the first sentence of this paragraph shall be adjusted
to, respectively, the total number of Founder Shares that the Sponsor would have to return to the Company in order for the number
of Founder Shares that the Sponsor owns (together with the Insiders) to equal an aggregate of 20.0% of the Company’s issued
and outstanding Shares after the Public Offering.

 

6. The
Sponsor and each Insider hereby agrees and acknowledges that: (i) the Underwriters and the Company would be irreparably injured
in the event of a breach by such Sponsor or Insider of its, his or her obligations under paragraphs 1, 2, 3, 4, 5, 7(a), 7(b),
and 9 of this Letter Agreement (ii) monetary damages may not be an adequate remedy for such breach and (iii) the non-breaching
party shall be entitled to seek injunctive relief, in addition to any other remedy that such party may have in law or in equity,
in the event of such breach.

 

7. (a)
The Sponsor and each Insider agrees that it, he or she shall not Transfer (as defined below) any Founder Shares (or shares of Class
A Common Stock issuable upon conversion thereof) until the earlier of (A) one year after the completion of the Company’s
initial Business Combination and (B) subsequent to the Business Combination, (x) the date on which the Company completes a liquidation,
merger, stock exchange, reorganization or other similar transaction that results in all of the Public Stockholders having the right
to exchange their shares of Common Stock for cash, securities or other property or (y) if the last reported sale price of the Class
A Common Stock equals or exceeds $12.00 per share (as adjusted for stock splits, stock dividends, reorganizations, recapitalizations
and the like) for any 20 trading days within any 30-trading day period commencing at least 150 days after the Company’s initial
Business Combination (the “Founder Shares Lock-Up Period”).

 

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(b) The
Sponsor agrees that it shall not Transfer any Private Placement Warrants or any shares of Class A Common Stock issued or issuable
upon the exercise of the Private Placement Warrants, until 30 days after the completion of a Business Combination (the “Private
Placement Warrants Lock-Up Period”, together with the Founder Shares Lock-Up Period, the “Lock-Up Periods”).

 

(c) Notwithstanding
the provisions set forth in paragraphs 7(a) and (b), Transfers of the Founder Shares, Private Placement Warrants and shares of
Class A Common Stock issued or issuable upon the exercise or conversion of the Private Placement Warrants or the Founder Shares
and that are held by the Sponsor or any Insider or any of their permitted transferees (that have complied with this paragraph 7(c)),
are permitted (a) to the Company’s directors or officers, any affiliates or family members of any of the Company’s
directors or officers, any members of the Sponsor, or any affiliates of the Sponsor, (b) in the case of an individual, by gift
to a member of the individual’s immediate family or to a trust, the beneficiary of which is a member of the individual’s
immediate family or an affiliate of such person, or to a charitable organization; (c) in the case of an individual, by virtue of
laws of descent and distribution upon death of the individual; (d) in the case of an individual, pursuant to a qualified domestic
relations order; (e) by private sales or transfers made in connection with the consummation of the Company’s Business Combination
at prices no greater than the price at which the securities were originally purchased; (f) in the event of the Company’s
liquidation prior to the Company’s completion of an initial Business Combination; (g) by virtue of the laws of Delaware or
the Sponsor’s limited liability company agreement, as amended, upon dissolution of the Sponsor; or (h) in the event of the
Company’s completion of a liquidation, merger, stock exchange, reorganization or other similar transaction which results
in all of the Public Stockholders having the right to exchange their shares of Class A Common Stock for cash, securities or other
property subsequent to the Company’s completion of an initial Business Combination; provided, however, that
in the case of clauses (a) through (e), these permitted transferees must enter into a written agreement with the Company agreeing
to be bound by the transfer restrictions and other applicable restrictions in this Letter Agreement.

 

8. The
Sponsor and each Insider represents and warrants that it, he or she has never been suspended or expelled from membership in any
securities or commodities exchange or association or had a securities or commodities license or registration denied, suspended
or revoked. Each Insider’s biographical information furnished to the Company, if any (including any such information included
in the Prospectus), is true and accurate in all respects and does not omit any material information with respect to such Insider’s
background. The Sponsor and each Insider’s questionnaire furnished to the Company, if any, is true and accurate in all respects.
The Sponsor and each Insider represents and warrants that: it is not subject to or a respondent in any legal action for, any injunction,
cease-and-desist order or order or stipulation to desist or refrain from any act or practice relating to the offering of securities
in any jurisdiction; it has never been convicted of, or pleaded guilty to, any crime (i) involving fraud, (ii) relating to any
financial transaction or handling of funds of another person, or (iii) pertaining to any dealings in any securities and it is not
currently a defendant in any such criminal proceeding.

 

9. Except
as disclosed in, or as expressly contemplated by, the Prospectus, neither the Sponsor nor any Insider nor any affiliate of the
Sponsor or any Insider, nor any director or officer of the Company, shall receive from the Company any finder’s fee, reimbursement,
consulting fee, monies in respect of any repayment of a loan or other compensation prior to, or in connection with any services
rendered in order to effectuate the consummation of the Company’s initial Business Combination (regardless of the type of
transaction that it is).

 

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10. The
Sponsor and each Insider has full right and power, without violating any agreement to which it is bound (including, without limitation,
any non-competition or non-solicitation agreement with any employer or former employer), to enter into this Letter Agreement and,
as applicable, to serve as an officer and/or a director on the board of directors of the Company and hereby consents to being named
in the Prospectus as an officer and/or a director of the Company.

 

11. As
used herein, (i) “Business Combination” shall mean a merger, capital stock exchange, asset acquisition,
stock purchase, reorganization or similar business combination, involving the Company and one or more businesses; (ii) “Shares”
shall mean, collectively, the shares of Class A Common Stock and the Founder Shares; (iii) “Founder Shares”
shall mean the 8,625,000 shares of Class B common stock, par value $0.0001 per share, issued and outstanding immediately prior
to the consummation of the Public Offering; (iv) “Initial Stockholders” shall mean the Sponsor and
any Insider that holds Founder Shares; (v) “Private Placement Warrants” shall mean the Warrants to purchase
an aggregate of 5,333,333 shares of Class A Common Stock of the Company (or up to 5,933,333 shares of Class A Common Stock depending
on the extent to which the Underwriters’ over-allotment option is exercised pursuant to the Underwriting Agreement) that
the Sponsor has agreed to purchase for an aggregate purchase price of $8,000,000 in the aggregate (or up to $8,900,000 depending
on the extent to which the Underwriters’ over-allotment option is exercised pursuant to the Underwriting Agreement), or $1.50
per Warrant, in a private placement that shall occur substantially concurrently with the consummation of the Public Offering; (vi) “Public
Stockholders” shall mean the holders of securities issued in the Public Offering; (vii) “Trust Account”
shall mean the trust fund into which a portion of the net proceeds of the Public Offering shall be deposited; and (viii) “Transfer”
shall mean the (a) sale or assignment of, offer to sell, contract or agreement to sell, hypothecate, pledge, grant of any
option to purchase or otherwise dispose of or agreement to dispose of, directly or indirectly, or establishment or increase of
a put equivalent position or liquidation with respect to or decrease of a call equivalent position within the meaning of Section
16 of the Securities Exchange Act of 1934, as amended, and the rules and regulations of the Commission promulgated thereunder with
respect to, any security, (b) entry into any swap or other arrangement that transfers to another, in whole or in part, any
of the economic consequences of ownership of any security, whether any such transaction is to be settled by delivery of such securities,
in cash or otherwise, or (c) public announcement of any intention to effect any transaction specified in clause (a) or (b)
herein.

 

12. This
Letter Agreement constitutes the entire agreement and understanding of the parties hereto in respect of the subject matter hereof
and supersedes all prior understandings, agreements, or representations by or among the parties hereto, written or oral, to the
extent they relate in any way to the subject matter hereof or the transactions contemplated hereby. This Letter Agreement may not
be changed, amended, modified or waived (other than to correct a typographical error) as to any particular provision, except by
a written instrument executed by (1) each Insider that is the subject of any such change, amendment modification or waiver and
(2) the Sponsor.

 

    	 	6	 

    	 	 

    

 

13. No
party hereto may assign either this Letter Agreement or any of its rights, interests, or obligations hereunder without the prior
written consent of the other parties. Any purported assignment in violation of this paragraph shall be void and ineffectual and
shall not operate to transfer or assign any interest or title to the purported assignee. This Letter Agreement shall be binding
on the Sponsor and each Insider and their respective successors, heirs and assigns and permitted transferees.

 

14. This
Letter Agreement shall be deemed severable, and the invalidity or unenforceability of any term or provision hereof shall not affect
the validity or enforceability of this Letter Agreement or of any other term or provision hereof. Furthermore, in lieu of any such
invalid or unenforceable term or provision, the parties hereto intend that there shall be added as a part of this Letter Agreement
a provision as similar in terms to such invalid or unenforceable provision as may be possible and be valid and enforceable.

 

15. This
Letter Agreement shall be governed by and construed and enforced in accordance with the laws of the State of New York. The parties
hereto (i) all agree that any action, proceeding, claim or dispute arising out of, or relating in any way to, this Letter Agreement
shall be brought and enforced in the courts of New York City, in the State of New York, and irrevocably submit to such jurisdiction
and venue, which jurisdiction and venue shall be exclusive and (ii) waive any objection to such exclusive jurisdiction and venue
or that such courts represent an inconvenient forum.

 

16. Any
notice, consent or request to be given in connection with any of the terms or provisions of this Letter Agreement shall be in writing
and shall be sent by express mail or similar private courier service, by certified mail (return receipt requested), by hand delivery
or facsimile or other electronic transmission.

 

17. Each
party hereto shall not be liable for any breaches or misrepresentations contained in this Letter Agreement by any other party to
this Letter Agreement (including, for the avoidance of doubt, any Insider with respect to any other Insider), and no party shall
be liable or responsible for the obligations of another party, including, without limitation, indemnification obligations and notice
obligations.

 

18. This
Letter Agreement shall terminate on the earlier of (i) the expiration of the Lock-Up Periods and (ii) the liquidation of the Company;
provided, however, that this Letter Agreement shall earlier terminate in the event that the Public Offering is not
consummated and closed by December 31, 2020; provided further that paragraph 4 of this Letter Agreement shall survive such
liquidation.

 

19. This
Letter Agreement may be executed in any number of original or facsimile counterparts and each of such counterparts shall for all
purposes be deemed to be an original, and all such counterparts shall together constitute but one and the same instrument.

 

[Signature page follows]

 

    	 	7	 

    	 	 

    

 

	 	Sincerely,
	 	 
	 	FIRSTMARK HORIZON SPONSOR LLC

 

	 	By: 	 
	 	 	Name: 
	 	 	Title:   Authorized Signatory 

 

	 	 
	 	Richard Heitzmann
	 	 
	 	 
	 	Amish Jani
	 	 
	 	 
	 	Daniel Gaisin
	 	 
	 	 
	 	Eric Cheung
	 	 
	 	 
	 	Jason Robins 
	 	 
	 	 
	 	Luis Ubiñas 
	 	 
	 	 
	 	Frederick Ball
	 	 
	 	 
	 	Allison Goldberg

 

[Signature Page to Letter Agreement]

 

    	 	 	 

    	 	 

    

 

Acknowledged and Agreed:

 

FirstMark Horizon
Acquisition Corp.

  

	By: 	 	 
	 	Name:	 
	 	Title:	 

 

 

[Signature Page to Letter Agreement]

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