<PAGE>   1
                                                                   EXHIBIT 10.1

                               JOINDER AGREEMENT

         JOINDER AGREEMENT dated as of March 2, 2000, 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:



<PAGE>   2

                  (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 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.

<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 ADVAN, INC.
                                     a Pennsylvania corporation



                                     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

<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.

Lamar Advertising of Penn, LLC, Issuee of Stock


By:      Lamar Company, L.L.C.
         Its sole and managing member


By:      Lamar Media Corp.
         Its sole and managing member


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


<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>
Lamar Advertising of Penn, LLC               Lamar Advan, Inc.         1,000           4             100
</TABLE>


<PAGE>   6

                  SCHEDULE OF ADDITIONAL SUBSIDIARY GUARANTORS


<TABLE>
<CAPTION>

GUARANTOR*                                                   DATE OF JOINDER AGREEMENT
----------                                                   -------------------------
<S>                                                          <C>
Lamar Advertising of Iowa, Inc.                              March 2, 2000
Interstate Logos, L.L.C.                                     April 17, 2000
</TABLE>


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

<PAGE>   7

        SUPPLEMENT TO LAMAR ADVERTISING OF IOWA, 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.               Lamar Advertising of Iowa,     1,000           7             100
                                        Inc.
</TABLE>


<PAGE>   8

            SUPPLEMENT TO INTERSTATE LOGS, L.L.C. 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 Media Corp.                    Interstate Logos, L.L.C.          1,000           1             100
</TABLE>