<PAGE>   1
                                                                    EXHIBIT 10.1

                                JOINDER AGREEMENT

     JOINDER AGREEMENT dated as of March 15, 2001, by the undersigned, (the
"Additional Subsidiary Guarantor"), in favor of The Chase Manhattan Bank, as
administrative agent for the Lenders party to the Credit Agreement referred to
below (in such capacity, together with its successors in such capacity, the
"Administrative Agent").

     Lamar Media Corp. (formerly Lamar Advertising Company), a Delaware
corporation (the "Borrower"), and certain of its subsidiaries (collectively, the
"Existing Subsidiary Guarantors" and, together with the Borrower, the "Securing
Parties") are parties to a Credit Agreement dated August 13, 1999 (as modified
and supplemented and in effect from time to time, the "Credit Agreement",
providing, subject to the terms and conditions thereof, for extensions of credit
(by means of loans and letters of credit) to be made by the lenders therein
(collectively, together with any entity that becomes a "Lender" party to the
Credit Agreement after the date hereof as provided therein, the "Lenders" and,
together with Administrative Agent and any successors or assigns of any of the
foregoing, the "Secured Parties") to the Borrower in an aggregate principal or
face amount not exceeding $1,000,000,000 (which, in the circumstances
contemplated by Section 2.01(d) thereof, may be increased to $1,400,000,000). In
addition, the Borrower may from time to time be obligated to one or more of the
Lenders under the Credit Agreement in respect of Hedging Agreements under and as
defined in the Credit Agreement (collectively, the "Hedging Agreements").

     In connection with the Credit Agreement, the Borrower, the Existing
Subsidiary Guarantors and the Administrative Agent are parties to the Pledge
Agreement dated September 15, 1999 (the "Pledge Agreement") pursuant to which
the Securing Parties have, inter alia, granted a security interest in the
Collateral (as defined in the Pledge Agreement) as collateral security for the
Secured Obligations (as so defined). Terms defined in the Pledge Agreement are
used herein as defined therein.

     To induce the Secured Parties to enter into the Credit Agreement, and to
extend credit thereunder and to extend credit to the Borrower under Hedging
Agreements, and for other good and valuable consideration the receipt and
sufficiency of which are hereby acknowledged, the Additional Subsidiary
Guarantor has agreed to become a party to the Credit Agreement and the Pledge
Agreement as a "Subsidiary Guarantor" thereunder, and to pledge and grant a
security interest in the Collateral (as defined in the Pledge Agreement).

     Accordingly, the parties hereto agree as follows:

     Section 1. Definitions. Terms defined in the Credit Agreement are used
herein as defined therein.

     Section 2. Joinder to Agreements. Effective upon the execution and delivery
hereof, the Additional Subsidiary Guarantor hereby agrees that it shall become
"Subsidiary Guarantor" under and for all purposes of the Credit Agreement and
the Pledge Agreement with all the rights and obligations of a Subsidiary
Guarantor thereunder. Without limiting the generality of the foregoing, the
Additional Subsidiary Guarantor hereby:

          (i) jointly and severally with the other Subsidiary Guarantors party
     to the Credit Agreement guarantees to each Secured Party and their
     respective successors and assigns the

<PAGE>   2

     prompt payment in full when due (whether at stated maturity, by
     acceleration or otherwise) of all Guaranteed Obligations in the same manner
     and to the same extent as is provided in Article III of the Credit
     Agreement;

          (ii) pledges and grants the security interests in all right, title and
     interest of the Additional Subsidiary Guarantor in all Collateral (as
     defined in the Pledge Agreement) now owned or hereafter acquired by the
     Additional Subsidiary Guarantor and whether now existing or hereafter
     coming into existence provided for by Article III of the Pledge Agreement
     as collateral security for the Secured Obligations and agrees that Annex 1
     thereof shall be supplemented as provided in Appendix A hereto;

          (iii) makes the representations and warranties set forth in Article IV
     of the Credit Agreement and in Article II of the Pledge Agreement, to the
     extent relating to the Additional Subsidiary Guarantor or to the Pledged
     Equity evidenced by the certificates, if any, identified in Appendix A
     hereto; and

          (iv) submits to the jurisdiction of the courts, and waives jury trial,
     as provided in Sections 10.09 and 10.10 of the Credit Agreement.

     The Additional Subsidiary Guarantor hereby instructs its counsel to deliver
the opinions referred to in Section 6.10(a)(iii) of the Credit Agreement to the
Secured Parties.

                                       2
<PAGE>   3

     IN WITNESS WHEREOF, the Additional Subsidiary Guarantor has caused this
Joinder Agreement to be duly executed and delivered as of the day and year first
above written.

                                       Lamar Hardy Outdoor Advertising, Inc.


                                       By: /s/ Keith A. Istre
                                           ----------------------------
                                           Keith A. Istre
                                           Vice President - Finance and
                                           Chief Financial Officer

Attested:


By: /s/ James R. McIlwain
    -------------------------------
    James R. McIlwain, Secretary

Accepted and agreed:

THE CHASE MANHATTAN BANK,
as Administrative Agent


By: /s/ William E. Rottino
    -------------------------------
Title: Vice President

                                       3
<PAGE>   4

The undersigned hereby respectively pledges and grants a security interest in
the Pledged Equity and evidenced by the certificate listed in Appendix A hereto
and agrees that Annex 1 of the above-referenced Pledge Agreement is hereby
supplemented by adding thereto the information listed on Appendix A.

The Lamar Company, L.L.C., Issuee

By: Lamar Media Corp.
Its: Managing Member

By: /s/ Keith A. Istre
    ---------------------------
       Keith A. Istre
Title: Vice President-Finance

                                       4
<PAGE>   5

SUPPLEMENT TO ANNEX 1

                                                 APPENDIX A TO JOINDER AGREEMENT

<TABLE>
<CAPTION>
PLEDGOR OWNERSHIP                            ISSUER                    NO. SHARES      CERT. NO.      %
-----------------                            ------                    ----------      ---------     ---

<S>                                          <C>                       <C>             <C>           <C>
The Lamar Company, L.L.C.                    Lamar Hardy Outdoor       100 Common      3             100
                                             Advertising, Inc.         Stock
</TABLE>

                                       5
<PAGE>   6

                  SCHEDULE OF ADDITIONAL SUBSIDIARY GUARANTORS

<TABLE>
<CAPTION>
GUARANTOR*                                                   DATE OF JOINDER AGREEMENT
----------                                                   -------------------------

<S>                                                          <C>
Lamar G&H Outdoor Advertising, L.L.C.                        March 15, 2001

Superior Outdoor Advertising, Inc.                           January 15, 2001

Custom Leasing Realty, Inc.                                  January 15, 2001

Arkansas Outdoor Advertising Co.                             January 15, 2001

Outdoor Marketing Systems, Inc.                              January 24, 2001

Outdoor Marketing Systems, L.L.C.                            January 24, 2001

Able Outdoor, Inc.                                           January 24, 2001
</TABLE>

* The supplements to Annex 1/Appendix A to the Joinder Agreements of each
additional guarantor are set forth below in their entirety.

                                       6
<PAGE>   7

      SUPPLEMENT TO LAMAR G&H OUTDOOR ADVERTISING, L.L.C. JOINDER AGREEMENT

SUPPLEMENT TO ANNEX 1

                                                 APPENDIX A TO JOINDER AGREEMENT

<TABLE>
<CAPTION>
PLEDGOR OWNERSHIP                       ISSUER                         NO. UNITS       CERT. NO.      %
-----------------                       ------                         ---------       ---------     ---
<S>                                     <C>                            <C>             <C>           <C>
Lamar Central Outdoor, Inc.             Lamar G&H Outdoor              1,000           1             100
                                        Advertising, L.L.C.
</TABLE>

                                       7
<PAGE>   8

       SUPPLEMENT TO SUPERIOR OUTDOOR ADVERTISING, INC. JOINDER AGREEMENT

SUPPLEMENT TO ANNEX 1

                                                 APPENDIX A TO JOINDER AGREEMENT

<TABLE>
<CAPTION>
PLEDGOR OWNERSHIP                    ISSUER                            NO. SHARES      CERT. NO.      %
-----------------                    ------                            ----------      ---------     ---
<S>                                  <C>                               <C>             <C>           <C>
Lamar Advantage Holding              Superior Outdoor                  10,000          2             100
Company                              Advertising, Inc.                 Common
                                                                       Stock
</TABLE>

                                       8
<PAGE>   9

          SUPPLEMENT TO CUSTOM LEASING & REALTY, INC. JOINDER AGREEMENT

SUPPLEMENT TO ANNEX 1

                                                 APPENDIX A TO JOINDER AGREEMENT

<TABLE>
<CAPTION>
PLEDGOR OWNERSHIP                    ISSUER                            NO. SHARES      CERT. NO.      %
-----------------                    ------                            ----------      ---------     ---
<S>                                  <C>                               <C>             <C>           <C>
Lamar Advantage Holding              Custom Leasing & Realty,          100 Common      5             100
Company                              Inc.                              Stock
</TABLE>

                                       9
<PAGE>   10

     SUPPLEMENT TO ARKANSAS OUTDOOR ADVERTISING CO., INC. JOINDER AGREEMENT

SUPPLEMENT TO ANNEX 1

                                                 APPENDIX A TO JOINDER AGREEMENT

<TABLE>
<CAPTION>
PLEDGOR OWNERSHIP                    ISSUER                            NO. SHARES      CERT. NO.      %
-----------------                    ------                            ----------      ---------     ---
<S>                                  <C>                               <C>             <C>           <C>
Lamar Advantage Holding              Arkansas Outdoor                  300 Common      3             100
Company                              Advertising Co., Inc.             Stock
</TABLE>

                                       10
<PAGE>   11

         SUPPLEMENT TO OUTDOOR MARKETING SYSTEMS, INC. JOINDER AGREEMENT

SUPPLEMENT TO ANNEX 1

                                                 APPENDIX A TO JOINDER AGREEMENT

<TABLE>
<CAPTION>
PLEDGOR OWNERSHIP                    ISSUER                            NO. SHARES      CERT. NO.     %
-----------------                    ------                            ----------      ---------     -
<S>                                  <C>                               <C>             <C>           <C>
The Lamar Company, L.L.C.            Outdoor Marketing                 10,000          5             100
                                     Systems, Inc.                     Common
                                                                       Stock
</TABLE>

                                       11
<PAGE>   12

        SUPPLEMENT TO OUTDOOR MARKETING SYSTEMS, L.L.C. JOINDER AGREEMENT

SUPPLEMENT TO ANNEX 1

                                                 APPENDIX A TO JOINDER AGREEMENT

<TABLE>
<CAPTION>
PLEDGOR OWNERSHIP                    ISSUER                            NO. UNITS       CERT. NO.      %
-----------------                    ------                            ---------       ---------     ---
<S>                                  <C>                               <C>             <C>           <C>
Outdoor Marketing Systems,           Outdoor Marketing                 1000            1             100
 Inc.                                Systems, L.L.C.
</TABLE>

                                       12
<PAGE>   13

               SUPPLEMENT TO ABLE OUTDOOR, INC. JOINDER AGREEMENT

SUPPLEMENT TO ANNEX 1

                                                 APPENDIX A TO JOINDER AGREEMENT

<TABLE>
<CAPTION>
PLEDGOR OWNERSHIP                    ISSUER                            NO. SHARES      CERT. NO.      %
-----------------                    ------                            ----------      ---------     ---
<S>                                  <C>                               <C>             <C>           <C>
The Lamar Company, L.L.C.            Able Outdoor, Inc.                1200            6             100
                                                                       Common
                                                                       Stock
</TABLE>

                                       13