<PAGE>
                                                                   Exhibit 10.21


                               SUBLEASE AGREEMENT

This Agreement is made and entered into this ___________ day of _______________,
2003 by and between GREAT AMERICAN INSURANCE COMPANY, an Ohio corporation with
its principal office located at 580 Walnut Street, Cincinnati, Ohio
("Sublessor"), and INFINITY PROPERTY AND CASUALTY CORPORATION, an Ohio
corporation with its principal office located at 2204 Lakeshore Drive,
Birmingham, Alabama 35209 ("Sublessee").

                                    RECITALS

A.    Sublessor entered into a certain lease with W9/LWS II Real Estate Limited
      Partnership ("Landlord") dated July 12, 2000 ("Master Lease"), a copy of
      which is attached hereto as Exhibit A and made a part hereof, wherein
      Sublessor rented space in the building located at 500 Lanidex Plaza,
      Parsippany, New Jersey (the "Building"). The space which is subject to the
      Master Lease is called the "Premises".

B.    Sublessor is transferring certain of its insurance business and Sublessee
      or an affiliate will thereafter be servicing certain of such business.
      Upon the closing of such transaction, it is contemplated that the Premises
      will be occupied by employees and agents of Sublessor ("Sublessor
      Employees") and by employees and agents of Sublessee ("Sublessee
      Employees"). The parties anticipate that the Sublessor Employees will
      constitute approximately 93.69% ("Sublessor's Percentage"), and the
      Sublessee Employees will constitute approximately 6.31% ("Sublessee's
      Percentage"), of the total of the Sublessor Employees and the Sublessee
      Employees which are sharing space in the Premises. The parties do not
      intend to divide the Premises as between Sublessor and Sublessee, with one
      portion to be used exclusively by Sublessor and the Sublessor Employees
      and the other portion to be used exclusively by Sublessee and the
      Sublessee Employees. Rather, the parties intend to enter into this
      Sublease to set forth the terms and conditions on which Sublessor and
      Sublessee will share the Premises during the term of this Sublease.

NOW, THEREFORE, in consideration of the mutual promises, covenants and
conditions hereinafter set forth, the parties agree as follows:

                              TERMS AND CONDITIONS

      1.    Premises: Sublessor hereby leases the Premises to Sublessee, and
Sublessee hereby leases the Premises from Sublessor for those uses set forth in
the Master Lease and for no other purpose. Notwithstanding the above provisions
of this Paragraph 1, Sublessee's right to possession of the Premises shall not
be exclusive, and shall be shared with Sublessor as provided in Paragraph 2.

      2.    Space-Sharing Arrangement: This Sublease is intended to accomplish a
space-sharing arrangement between Sublessor and Sublessee during the term of
this Sublease, rather than a sublease of a specific portion of the Premises,
with Sublessor and Sublessee sharing the occupancy costs in accordance with the
Sublessor's Percentage and the Sublessee's Percentage, respectively. Sublessor
and Sublessee will cooperate with each other and will act in good faith
<PAGE>
in order to coordinate their shared use of the Premises so each can conduct its
business without unreasonable interference from the other during the term of
this Sublease.

      3.    Term: The term of this Sublease shall commence on _______________,
2003 and shall expire _____________________, 2005. This Sublease shall
automatically expire if the Master Lease expires or is terminated.

      4.    Rental: Sublessee hereby agrees to pay to Sublessor Sublessee's
Percentage of the same rent as computed, required and set forth in the Master
Lease, payable in monthly installments due and payable thirty (30) days after
Sublessee's receipt of Sublessor's statement at Sublessor's office at 580 Walnut
Street, Cincinnati, Ohio 45202 or at such other place as Sublessor may from time
to time designate by written notice to Sublessee.

      5.    Renewal: Provided the Master Lease will, according to its terms, be
in effect for the full renewal term, Sublessee shall have the right to renew
this Sublease for one additional term of two years. Sublessee shall exercise its
renewal rights by written notice to Sublessor not later than one hundred twenty
(120) days prior to the end of the then expiring term.

      6.    Representations: Sublessor hereby warrants and represents that it is
now leasing the Premises pursuant to the terms and provisions set forth in the
Master Lease, that a true and complete copy of the Master Lease is attached
hereto as Exhibit A, that the Master Lease is in full force and effect, and that
Sublessor has a valid leasehold interest in the Premises under the Master Lease;
that neither the Master Lease nor any of the obligations, duties, and
responsibilities of the Sublessor or of the Landlord under the Master Lease
("Master Landlord") have been amended, modified, or altered in any manner
whatsoever; and that there exists no circumstance, condition or act of default
which would entitle or permit the Master Landlord to terminate the Master Lease
or to abridge any rights of Sublessor as Tenant thereunder. Sublessor covenants
that it will not modify or surrender the Master Lease without the prior written
consent of Sublessee, and represents that is has full right, power and authority
under the Master Lease and otherwise to enter into this Sublease.

      7.    Master Lease:

            A.    All the obligations contained in the Master Lease conferred
      and imposed upon Sublessor (as Tenant therein) shall be borne by Sublessor
      and Sublessee in accordance with the Sublessor's Percentage and the
      Sublessee's Percentage, respectively, except as modified and amended by
      this Sublease, and all rights and privileges contained in the Master Lease
      conferred upon Sublessor (as Tenant therein), are hereby conferred and
      imposed upon Sublessee, to the extent of Sublessee's Percentage. Sublessor
      covenants and agrees it will make payment of the rentals reserved under
      the Master Lease as and when due, will perform Sublessor's insurance
      obligations under the Master Lease, and will otherwise fully and
      faithfully perform the terms and conditions of the Master Lease with
      respect to the Sublessor's Percentage. Sublessee covenants and agrees to
      otherwise fully and faithfully perform the terms and conditions of the
      Master Lease and the Sublease on its part to be performed. Neither the
      Sublessor nor Sublessee shall do or cause to be done any act which would
      or might cause the Master Lease, or the rights of Sublessor as tenant
      under the Master Lease to be endangered, cancelled, terminated,


                                       2
<PAGE>
      forfeited or surrendered, or which would or might cause Sublessor to be in
      default thereunder or liable for any damage, claim or penalty. Sublessee
      agrees, as an express inducement for Sublessor executing this Sublease,
      that if there is any conflict between the provisions of the Master Lease
      and this Sublease which would permit Sublessee to do or cause to be done
      any act which is prohibited by the Master Lease then the provisions of the
      Master Lease shall prevail.

            B.    Sublessee shall pay to Sublessor, within ten (10) days after
      demand therefor by Sublessor, Sublessee's Percentage of any and all sums
      (except fixed annual rent payable under the Master Lease) due pursuant to
      the Master Lease. Sublessor shall not demand such payment prior to the
      date which is thirty (30) days before the date any such sum shall be due
      and owing under the Master Lease.

            C.    Notwithstanding anything to the contrary herein contained,
      Sublessor shall have no duty itself to perform any obligations of the
      Master Landlord, nor shall such default of the Master Landlord affect this
      Sublease or waive or defer the performance of any of Sublessee's
      obligations hereunder; provided, nevertheless, that in the event of any
      such default or failure of performance by Master Landlord, Sublessor
      agrees, upon notice from Sublessee, to make immediate demand upon Master
      Landlord to perform its obligations under the Master Lease.

      8.    Default:

            A.    If Sublessee defaults in the performance of any of its
      obligations hereunder, and such default continues for five (5) days after
      the giving of notice of such default with respect to the failure to pay
      any monies, or ten (10) days after the giving of notice of default with
      respect to the failure to perform or comply with any non-monetary
      obligations of Sublessee hereunder, then Sublessor may cure any such
      default and add the cost thereof (including reasonable attorneys' fees) to
      rent or terminate this Sublease upon giving three (3) days notice of
      termination to Sublessee. Sublessee shall have reasonable additional time
      beyond ten (10) days to cure a non-monetary default if Sublessee has
      commenced to cure same within said ten (10) days and thereafter proceeds
      with due diligence to cure same.

            B.    Notwithstanding anything to the contrary contained herein,
      Sublessor may terminate this Sublease on three (3) days notice of
      termination (without having given prior notice of default) if in the
      reasonable apprehension of Sublessor the act or omission of Sublessee
      would cause a default under the Master Lease such as would entitle the
      Master Landlord within said period to terminate the Master Lease.

            C.    In the event of Sublessee's default hereunder Sublessee shall
      remain liable for all rent and other sums due under the Sublease for the
      remainder of what would have been the term, for all damages arising out of
      its default and for all costs incurred in connection with any re-letting
      of the Premises. Sublessor shall have the right to re-enter and take
      possession of the Premises in the event of a termination for default. In
      addition to any and all remedies set forth herein Sublessor shall have all
      remedies available at law


                                       3
<PAGE>
      or in equity and any and all remedies shall be cumulative and
      non-exclusive. Sublessor shall in any event, remain liable for all
      obligations imposed upon it by the Master Lease.

      9.    No Representations: Sublessor makes no representations with respect
to this transaction or the Premises, except as specifically set forth herein,
and Sublessee expressly acknowledges that no such representations have been
made. Sublessee takes the Premises in their "as is" condition, subject to
reasonable wear and tear. Sublessor warrants that the Premises shall, at the
time possession is delivered to Sublessee, be in substantially the same
condition as the date of execution of this Sublease.

      10.   Mechanics Liens: Sublessee shall cause no mechanics liens to be
placed against the Premises or any portion thereof.

      11.   Surrender: Sublessee shall surrender the Premises upon termination
of this Sublease in the same condition as on the date of commencement, subject
to ordinary wear and tear, casualty loss and damage caused by Sublessor or
Sublessor's Employees.

      12.   Notices: All notices, demands, submissions and consents required
hereunder shall be in writing and shall be deemed given if sent by certified
mail, return receipt requested postage prepaid (a) to Sublessee, at the address
of Sublessee as hereinabove set forth or such other address as Sublessee may
designate by notice to Sublessor, or (b) to Sublessor, at the address
hereinabove set forth, or such other address as Sublessor may designate by
notice to Sublessee.

      13.   Liability Insurance: Sublessor is presently maintaining, and will
continue to maintain, general public liability insurance with respect to the
Premises for amounts in excess of $1,000,000 up to $25,000,000 per occurrence,
and will cause Sublessee to be an additional named insured under such policy up
to $25,000,000. Sublessee will reimburse Sublessor for Sublessee's Percentage
of the premiums for such insurance that are attributable to the term of this
Sublease. Sublessee will defend, indemnify and hold harmless Sublessor for any
loss, damage or liability relating to the Premises caused by Sublessee or
Sublessee's agents, contractors, employees, invitees or licensees, up to
$1,000,000. In addition, Sublessee will indemnify Sublessor for any loss, damage
or liability caused by Sublessee not covered by Sublessor's general public
liability insurance.

      14.   Contents Insurance: Sublessor is presently maintaining, and will
continue to maintain, casualty insurance with respect to the personal property
located in the Premises, with a deductible not greater than $100,000 per
occurrence, and will cause Sublessee to be named as a loss payee under such
policy. Sublessee will reimburse Sublessor for Sublessee's Percentage of the
premiums for such insurance that are attributable to the term of this Sublease.
Sublessee will indemnify Sublessor against loss or damage to the personal
property located on the Premises caused by Sublessee or Sublessee's Employees,
up to the amount of the insurance deductible (which shall not exceed $100,000
per occurrence).



                                       4
<PAGE>
      15.   Indemnification:

            A.    Sublessee's Indemnity of Sublessor. Sublessee shall indemnify
      and hold Sublessor harmless against any and all claims, liabilities,
      damages or losses, and any attorneys fees and other incidental expenses,
      resulting from injury or death of any person or damage to property
      occurring on or about the Premises or arising in conjunction with the use
      and occupancy of the Premises by Sublessee or others claiming under
      Sublessee.

            B.    Sublessor's Indemnity of Sublessee. Sublessor shall indemnify
      and hold Sublessee harmless against any and all claims, liabilities,
      damages or losses, and any attorneys fees and other incidental expenses,
      resulting from injury or death of any person or damage to property
      occurring on or about the Premises or arising in conjunction with the use
      and occupancy of the Premises by Sublessor or others claiming under
      Sublessor.

      16.   Alterations: Sublessee shall not make any alterations or additions
to the Premises without first obtaining Sublessor's and Master Landlord's
consent.

      17.   Assignment: Without the prior written consent of Sublessor, and
Master Landlord, neither Sublessee, nor Sublessee's legal representatives or
successors in interest by operation of law or otherwise, shall assign or
mortgage this Sublease, or sublet or license the whole or any part of the
Premises or permit the Premises or any part thereof to be used or occupied by
others. Any consent by Sublessor and Master Landlord to any act of assignment or
subletting shall be held to apply only to the specific transaction thereby
authorized. Such consent shall not be construed as waiver of the duty of
Sublessee, or the legal representatives or assigns of Sublessee, to obtain from
Sublessor or Master Landlord consent to any other or subsequent assignment or
subletting, or as modifying or limiting the rights of Sublessor or Master
Landlord to assign or sublet without such consent. Both Sublessor and Sublessee
shall assume and be liable to Master Landlord for any and all acts and omissions
of any and all assignees, subtenants, undertenants and occupants.

      18.   Consents: With respect to any provision of this Sublease which
provides, in effect, that Sublessor shall not unreasonably withhold or
unreasonably delay any consent or any approval, Sublessee, shall not in any
event be entitled to make, nor shall Sublessee make, any claim for money damages
and Sublessee hereby waives any claim for money damages; nor shall Sublessee
claim any money damages by way of setoff, counterclaim or defense, based upon
any claim or assertion by Sublessee that Sublessor has unreasonably withheld or
unreasonably delayed any consent or approval; but Sublessee's sole remedy shall
be an action or proceeding to enforce any such provisions, or for specific
performance, injunction or declaratory judgment.

      19.   No Broker: Sublessee covenants, warrants and represents that there
was no broker instrumental in consummating this Sublease and that no
conversations or prior negotiations were had with any broker and agrees to
defend, indemnify and hold Sublessor harmless against any claims for brokerage
commission arising out of any conversations or negotiations had by Sublessee
with any broker.

      20.   Liability of Sublessor: In the event of a transfer of Sublessor's
interest in the Master Lease, or in this Sublease, it shall be deemed without
further agreement between the


                                       5
<PAGE>
parties and such transferee that the transferee has assumed and agreed to
observe and perform all obligations of the Sublessor hereunder. Notwithstanding
any such transfer, Sublessor shall be and remain liable to Sublessee for the
observance and performance of all obligations of the Sublessor hereunder, and
for breach of any of the representations and warranties made by Sublessor
herein.

      21.   Quiet Enjoyment: Sublessor warrants and agrees that, provided
Sublessee pays and performs all its covenants, agreements and obligations under
this Sublease, Sublessee shall have the quiet and peaceful enjoyment of the
Premises for the full term of this Sublease.

      22.   Waiver: One or more waivers of any covenants or conditions by
Sublessor shall not be construed as a waiver of a subsequent breach of the same
or any other covenants or conditions, and the consent or approval by Sublessor
to or of any act by Sublessee requiring Sublessor's consent or approval shall
not be construed to waive or render unnecessary Sublessor's consent or approval
to or of any subsequent similar act by Sublessee.

      23.   Effect: This Agreement shall be binding upon the parties hereto,
their heirs, successors and permitted assigns, and may not be altered, amended,
terminated or modified except by written instrument executed by each of the
parties hereto.

      24.   Forum: This Agreement shall be governed by the laws of the State
wherein the Premises are located.

IN WITNESS WHEREOF, the parties have executed this Agreement the date first
above written.

WITNESS                               GREAT AMERICAN INSURANCE COMPANY


                                      By:
------------------------------           ---------------------------------------
                                            Name:
                                                 -------------------------------
                                            Title:
                                                  ------------------------------


WITNESS                               INFINITY PROPERTY AND CASUALTY CORPORATION

                                      By:
------------------------------           ---------------------------------------
                                            Name:
                                                 -------------------------------
                                            Title:
                                                  ------------------------------



                                       6