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Exhibit 10.1

June 19, 2019

South Mountain Merger Corp.
767 Fifth Avenue, 9th Floor
New York, NY 10153

Citigroup Global Markets Inc.
388 Greenwich Street
New York, New York 10013

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”) to be entered
into by and among South Mountain Merger Corp., a Delaware corporation (the
“Company”) and Citigroup Global Markets Inc. (the “Underwriter”), relating to an
underwritten initial public offering (the “Public Offering”), of 25,875,000 of
the Company’s units (including up to 3,375,000 units that may be purchased to
cover over-allotments, if any) (the “Units”), each comprised of one share of the
Company’s Class A common stock, par value $0.0001 per share (the “Common
Stock”), and one half of one redeemable warrant.  Each whole Warrant (each, a
“Warrant”) entitles the holder thereof to purchase one share of Common Stock at
a price of $11.50 per share, subject to adjustment.  The Units will 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 U.S. Securities and
Exchange Commission (the “Commission”) and the Company has applied to have the
Units listed on the Nasdaq.  Certain capitalized terms used herein are defined
in paragraph 11 hereof.

In order to induce the Company and the Underwriter 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, South Mountain LLC (the “Sponsor”) and each of the undersigned
individuals, each of whom is a member of the Company’s board of directors and/or
management team (each, an “Insider” and collectively, the “Insiders”, which
term, for the avoidance of doubt, shall not include the Sponsor or any members
or managers of the Sponsor other than those members or managers who are also
members of the Company’s board of directors and/or management team)), hereby
severally (and not jointly and severally) agrees 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 of Capital Stock owned by it, him or her in favor of any proposed
Business Combination and (ii) not redeem any shares of Common Stock 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 “Charter”), 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 10 business days thereafter, subject to lawfully available
funds therefor, redeem 100% of the 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 (net of Permitted Withdrawals (as defined in the Prospectus) and net of
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 liquidating distributions, if any), subject to applicable law, 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 other
requirements of applicable law.  The Sponsor and each Insider agree to not
propose any amendment to the Charter that would modify the substance or timing
of the Company’s obligation to provide for the redemption of its public shares
in connection with a Business Combination or to redeem 100% of the Offering
Shares if the Company does not complete a Business Combination within 24 months
from the closing of the Public Offering, 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 (net
of Permitted Withdrawals), divided by the number of then outstanding Offering
Shares.

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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, him or her.  The Sponsor
and each Insider hereby further waive, with respect to any shares of Common
Stock held by it, him or her, if any, any redemption rights it, he or she may
have in connection with the consummation of a Business Combination, including,
without limitation, any such rights available in the context of a stockholder
vote to approve (a) Business Combination, (b) an amendment to the Charter that
would affect the substance or timing of the Company’s obligation to provide for
the redemption of our public shares in connection with a Business Combination or
to redeem 100% of the Class A Ordinary Shares if the Company has not consummated
a Business Combination within 24 months from the closing of the IPO, (c) in the
context of a tender offer made by the Company to purchase shares of Common Stock
(although the Sponsor, the Insiders and their respective affiliates 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 or in
connection with a stockholder vote to approve an amendment to the Charter to
modify the substance or timing of the Company’s obligation to redeem 100% of the
Offering Shares if the Company does not complete a Business Combination within
24 months from the closing of the Public Offering).
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3.                   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 Underwriter, (i)
sell, offer to sell, contract or agree to sell, hypothecate, pledge, grant any
option to purchase or otherwise dispose of or agree to dispose of, directly or
indirectly, or establish or increase a put equivalent position or liquidate or
decrease a call equivalent position within the meaning of Section 16 of the
Securities Exchange Act of 1934, as amended (the “Exchange Act”), and the rules
and regulations of the Commission promulgated thereunder, with respect to any
Units, shares of Capital Stock, Warrants or any securities convertible into, or
exercisable, or exchangeable for, shares of Common Stock owned by it, him or
her, (ii) enter into any swap or other arrangement that transfers to another, in
whole or in part, any of the economic consequences of ownership of any Units,
shares of Capital Stock, Warrants or any securities convertible into, or
exercisable, or exchangeable for, shares of Common Stock owned by it, him or
her, whether any such transaction is to be settled by delivery of such
securities, in cash or otherwise, or (iii) publicly announce any intention to
effect any transaction specified in clause (i) or (ii).  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 with respect to Insiders other than the Sponsor, below, the Company shall
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.  Any such release or waiver granted shall only be effective two business
days after the publication date of such press release.  The provisions of this
paragraph will not apply if (i) the release or waiver is effected solely to
permit a transfer of securities without 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
shareholders, members or managers of the Sponsor or any other Insider) 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 accountants) for
services rendered or products sold to the Company or (ii) any prospective target
business with which the Company has entered into a letter of intent,
confidentiality or other similar agreement for a Business Combination (a
“Target”); provided, however, that such indemnification of the Company by the
Sponsor (x) shall apply only to the extent necessary to ensure that such claims
by a third party (other than the Company’s independent public accountants) or a
Target do not reduce the amount of funds in the Trust Account to below the
lesser of (i) $10.00 per Offering Share or (ii) the actual amount per Offering
Share held in the Trust Account as of the date of the liquidation of the Trust
Account, if less than $10.00 per Offering Share is then held in the Trust
Account due to reductions in the value of the trust assets less interest earned
on the Trust Account which may be withdrawn to pay taxes, (y) shall not apply to
any claims by a third party (including a Target) that executed a waiver of any
and all rights to the monies held in the Trust Account (whether or not such
waiver is enforceable) and (z) shall not apply to any claims under the Company’s
indemnity of the Underwriter 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. 
For the avoidance of doubt, none of the Company’s officers or directors will
indemnify the Company for claims by third parties, including, without
limitation, claims by vendors and prospective target businesses.
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5.                   To the extent that the Underwriter does not exercise its
over-allotment option to purchase up to an additional 3,375,000 Units within 45
days from the date of the Prospectus (and as further described in the
Prospectus), the Sponsor agrees to forfeit, at no cost, a number of Founder
Shares in the aggregate equal to the product of 843,750 multiplied by a
fraction, (i) the numerator of which is 3,375,000 minus the number of Units
purchased by the Underwriter upon the exercise of its over-allotment option, and
(ii) the denominator of which is 3,375,000.  The forfeiture will be adjusted to
the extent that the over-allotment option is not exercised in full by the
Underwriter so that the Initial Stockholders will own an aggregate of 20.0% of
the Company’s issued and outstanding shares of Capital Stock after the Public
Offering.  To the extent that the size of the Public Offering is increased or
decreased, the Company will effect a capitalization or share repurchase,
redemption or stock split or other appropriate mechanism, as applicable,
immediately prior to the consummation of the Public Offering in such amount as
to maintain the ownership of the Capital Stock of the Initial Stockholders prior
to the Public Offering at 20.0% of the Company’s issued and outstanding Capital
Stock upon the consummation of the Public Offering.  In connection with such
increase or decrease in the size of the Public Offering, (A) references to
3,375,000 in the numerator and denominator of the formula in the first sentence
of this paragraph shall be changed to a number equal to 15% of the number of
shares included in the Units issued in the Public Offering and (B) the reference
to 843,750 in the formula set forth in the immediately preceding sentence shall
be adjusted to such number of Founder Shares that the Sponsor would have to
return to the Company in order to hold (with all of the Initial Stockholders) an
aggregate of 20.0% of the Company’s issued and outstanding Capital Stock after
the Public Offering.

6.                   (a)          The Company’s officers and directors each
hereby agree not to participate in the formation of, or become an officer or
director of, any other special purpose acquisition company with a class of
securities registered under the Exchange Act until the Company has entered into
a definitive agreement regarding a Business Combination or the Company has
failed to complete a Business Combination within the time period set forth in
the Charter.

(b)               The Sponsor and each Insider hereby agrees and acknowledges
that: (i) the Underwriter and the Company would be irreparably injured in the
event of a breach by such Sponsor or an Insider of its, his or her obligations
under paragraphs 1, 2, 3, 4, 5, 6(a), 7(a), 7(b), and 9, as applicable, 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 injunctive relief,
in addition to any other remedy that such party may have in law or in equity, in
the event of such breach.
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7.                   (a)          The Sponsor and each Insider agree that it or
he shall not Transfer any Founder Shares (or shares of Common Stock issuable
upon conversion thereof) until the earlier of (A) one year after the completion
of the Company’s initial Business Combination or (B) subsequent to the Business
Combination, (x) if the last sale price of the 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 or (y) the date on which the Company completes a
liquidation, merger, capital stock exchange, reorganization or other similar
transaction that results in all of the Company’s stockholders having the right
to exchange their shares of Common Stock for cash, securities or other property
(the “Founder Shares Lock-up Period”).

(b)               The Sponsor and each Insider agree that it, he or she shall
not Transfer any Private Placement Warrants (or shares of 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 3 and
7(a) and (b), Transfers of the Founder Shares, Private Placement Warrants and
shares of 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, any Insider or any of their permitted transferees (that have complied
with this paragraph 7(c)), are permitted (a) to the Company’s officers or
directors, any affiliates or family members of any of the Company’s officers or
directors, any members of the Sponsor, any affiliates of the Sponsor, or any
employees of such affiliates; (b) in the case of an individual, transfers by
gift to a member of the individual’s immediate family, 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, transfers by virtue of laws of descent and distribution upon death
of the individual; (d) in the case of an individual, transfers pursuant to a
qualified domestic relations order; (e) transfers by private sales or transfers
made at prices no greater than the price at which the securities were originally
purchased; (f) transfers in the event of the Company’s liquidation prior to the
completion of an initial Business Combination; (g) transfers by virtue of the
laws of the State of Delaware or the Sponsor’s limited liability company
agreement upon dissolution of the Sponsor; (h) to a nominee or custodian of a
person or entity to whom a disposition or transfer would be permissible under
clauses (a) through (g) above; provided, however, that in the case of clauses
(a) through (e) and (h), these permitted transferees must enter into a written
agreement with the Company agreeing to be bound by the transfer restrictions
herein.

8.                   The Sponsor and each Insider represent and warrant 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 (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 the Insider’s
background.  The Sponsor and each Insider’s questionnaire furnished to the
Company is true and accurate in all respects.  The Sponsor and each Insider
represents and warrants that: it, he or she 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, he or she 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, he or she is not currently
a defendant in any such criminal proceeding.
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9.                   Except as disclosed in the Prospectus, neither any 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), other than the
following, none of which will be made from the proceeds held in the Trust
Account prior to the completion of the initial Business Combination: repayment
of a loan and advances of up to $300,000 made to the Company by the Sponsor to
cover expenses related to the organization of the Company and the Public
Offering; payment to an affiliate of the Sponsor for office space and/or support
services for a total of $25,000 per month; reimbursement for any reasonable
out-of-pocket expenses related to identifying, investigating and consummating an
initial Business Combination, and repayment of loans, if any, and on such terms
as to be determined by the Company from time to time, made by the Sponsor or
certain of the Insiders to finance transaction costs in connection with an
intended initial Business Combination, provided, that, if the Company does not
consummate an initial Business Combination, a portion of the working capital
held outside the Trust Account may be used by the Company to repay such loaned
amounts so long as no proceeds from the Trust Account are used for such
repayment. Up to $1,500,000 of such loans may be convertible into warrants of
the post Business Combination entity at a price of $1.00 per warrant at the
option of the lender. Such warrants would be identical to the Private Placement
Warrants.

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. In the event of the removal or resignation of the Insider as a director
or officer (as applicable), the Insider agrees that he or she will not, prior to
the consummation of the Business Combination, without the prior express written
consent of the Company, (i) use for the benefit of the Insider or to the
detriment of the Company or (ii) disclose to any third party (unless required by
law or governmental authority), any information regarding a potential target of
the Company that is not generally known by persons outside of the Company, the
Sponsor, or their respective affiliates.
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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) “Capital Stock” shall mean, collectively, the Common Stock and the Founder
Shares; (iii) “Founder Shares” shall mean the 6,468,750 shares of the Company’s
Class B common stock, par value $0.0001 per share, (or 5,625,000 shares if the
over-allotment option is not exercised by the Underwriter) initially held by the
Sponsor; (iv) “Initial Stockholders” shall mean the Sponsor and any other holder
of Founder Shares immediately prior to the Public Offering; (v) “Private
Placement Warrants” shall mean the warrants to purchase up to 6,454,500 shares
of Common Stock of the Company (or 7,129,500 shares of Common Stock if the
over-allotment option is exercised in full) that the Sponsor has agreed to
purchase for an aggregate purchase price of $6,454,500 in the aggregate (or
$7,129,500 if the over-allotment option is exercised in full), or $1.00 per
warrant, in a private placement that shall occur simultaneously 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 and the sale of the Private Placement Warrants 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 Exchange Act 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).

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 all parties hereto.

13.          Except as otherwise provided herein, 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.
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14.          Nothing in this Letter Agreement shall be construed to confer upon,
or give to, any person or entity other than the parties hereto any right, remedy
or claim under or by reason of this Letter Agreement or of any covenant,
condition, stipulation, promise or agreement hereof.  All covenants, conditions,
stipulations, promises and agreements contained in this Letter Agreement shall
be for the sole and exclusive benefit of the parties hereto and their
successors, heirs, personal representatives and assigns and permitted
transferees.

15.          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.

16.          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.

17.          This Letter Agreement shall be governed by and construed and
enforced in accordance with the laws of the State of New York, without giving
effect to conflicts of law principles that would result in the application of
the substantive laws of another jurisdiction.  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.

18.          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 transmission.

19.          This Letter Agreement shall terminate on the earlier of (i) the
expiration of the Lock-up Periods or (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,
2019; provided further that paragraph 4 of this Letter Agreement shall survive
such liquidation for a period of six years.

[Signature Page Follows]
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Sincerely,
       
SOUTH MOUNTAIN MERGER CORP.
       
By:
/s/ Charles B. Bernicker  
Name:
Charles B. Bernicker
 
Title:
Chief Executive Officer

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Acknowledged and Agreed:

AGREED TO AND ACCEPTED BY:

SOUTH MOUNTAIN LLC

By: Harbour Reach Holdings LLC, its managing member

By: Netherton Investments Limited, its managing member

 
By: /s/ Robert Heaselgrave
   
Name: Robert Heaselgrave
   
Title:   Director
 

DIRECTORS AND MANAGEMENT TEAM OF SOUTH MOUNTAIN MERGER CORP.

/s/ Charles B. Bernicker  
Charles B. Bernicker
 
Chief Executive Officer
      /s/ Nicholas Dermatas  
Nicholas Dermatas
 
Chief Financial Officer and Secretary
      /s/ Robert L. Metzger  
Robert L. Metzger
 
Director
      /s/ Douglas J. Pauls  
Douglas J. Pauls
 
Director
 

[Signature Page to Letter Agreement]

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