<PAGE>   1
                                                                   EXHIBIT 10.23

                                JOINDER AGREEMENT

         JOINDER AGREEMENT dated as of December 23, 1999, 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:


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                  (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.


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         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 FLORIDA, INC.
                                        a Florida 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



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The undersigned hereby respectively pledge and grant a security interest in the
stock and evidenced by the certificate listed in Appendix A hereto and agree
that Annex 1 of the above-referenced Amended and Restated Pledge Agreement is
hereby supplemented by adding thereto the information listed on Appendix A.

Lamar Advertising of Mobile, Inc., Issuee of Stock



By:  /s/ Keith A. Istre                 Dated this day: December 23, 1999
     ------------------
      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 Mobile, Inc.     Lamar Florida, Inc.       1000         2       100
</TABLE>