<PAGE>
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                            ------------------------

                                    FORM 8-K
                                 CURRENT REPORT

                        PURSUANT TO SECTION 13 OR 15(D)
                     OF THE SECURITIES EXCHANGE ACT OF 1934

                       Date of report: September 21, 2000
                       (date of earliest event reported)

                             MACK-CALI REALTY, L.P.
--------------------------------------------------------------------------------
             (Exact name of Registrant as specified in its charter)

                                    DELAWARE
--------------------------------------------------------------------------------
                 (State or other jurisdiction of incorporation)

<TABLE>
<S>                                                       <C>
               333-57103                                                22-3315804
--------------------------------------                    --------------------------------------
         (Commission File No.)                                       (I.R.S. Employer
                                                                    Identification No.)
</TABLE>

                 11 COMMERCE DRIVE, CRANFORD, NEW JERSEY 07016
--------------------------------------------------------------------------------
              (Address of Principal Executive Offices) (Zip Code)

                                 (908) 272-8000
                                  ------------
              (Registrant's telephone number, including area code)

                                      N/A
--------------------------------------------------------------------------------
         (Former Name or Former Address, if Changed Since Last Report)

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
<PAGE>
ITEM 5.  OTHER EVENTS

    Mack-Cali Realty, L.P., a Delaware limited partnership ("Mack-Cali
Partnership"), is managed by Mack-Cali Realty Corporation, a Maryland
corporation (the "Company"), as its sole general partner. On September 22, 2000,
the Company and Prentiss Properties Trust, a Maryland real estate investment
trust ("Prentiss"), announced that they had mutually agreed to terminate the
Agreement and Plan of Merger (the "Merger Agreement") dated as of June 27, 2000
among the Company, Mack-Cali Partnership, Prentiss and Prentiss Properties
Acquisition Partners, L.P., a Delaware limited partnership of which Prentiss
(through a wholly-owned direct subsidiary) is the sole general partner
("Prentiss Partnership"). In connection with such termination, the Company
deposited $25 million into escrow for the benefit of Prentiss and Prentiss
Partnership.

    The Company and Prentiss also announced that they had simultaneously
consummated a purchase and sale transaction whereby the Company sold to Prentiss
its approximately 270,000 square foot Cielo Center property located in Austin,
Texas for a purchase price of $47,175,000.

    In connection with the foregoing, Mack-Cali Partnership hereby files the
following documents.

ITEM 7.  FINANCIAL STATEMENTS, PRO FORMA FINANCIAL INFORMATION AND EXHIBITS

    (c) Exhibits.

<TABLE>
            <S>           <C>
            10.1          Termination and Release Agreement, dated September 21, 2000,
                          by and among Mack-Cali Realty Corporation, Mack-Cali Realty,
                          L.P., Prentiss Properties Trust and Prentiss Properties
                          Acquisition Partners, L.P.

            10.2          Escrow Agreement, dated September 21, 2000, by and among
                          Mack-Cali Realty Corporation, Mack-Cali Realty, L.P.,
                          Prentiss Properties Trust, Prentiss Properties Acquisition
                          Partners, L.P. and Chicago Title Insurance Company, as
                          escrow agent.

            10.3          Purchase and Sale Contract, dated September 21, 2000, by and
                          between Mack-Cali Texas Property L.P. and Prentiss
                          Properties Acquisition Partners, L.P.

            99.1          News Releases dated September 22, 2000.
</TABLE>

                                       2
<PAGE>
                                   SIGNATURES

    Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned hereunto duly authorized.

<TABLE>
<S>                                                    <C>  <C>
                                                       MACK-CALI REALTY, L.P.

                                                       By:  Mack-Cali Realty Corporation,
                                                            its general partner

Date: September 22, 2000                               By:  /s/ ROGER W. THOMAS
                                                            -----------------------------------------
                                                            Roger W. Thomas
                                                            Executive Vice President, General Counsel
                                                            and Secretary
</TABLE>

                                       3
<PAGE>
                                 EXHIBIT INDEX

<TABLE>
<CAPTION>
EXHIBIT                                           DOCUMENT
-------                                           --------
<C>                     <S>
         10.1           Termination and Release Agreement, dated September 21, 2000,
                        by and among Mack-Cali Realty Corporation, Mack-Cali Realty,
                        L.P., Prentiss Properties Trust and Prentiss Properties
                        Acquisition Partners, L.P.

         10.2           Escrow Agreement, dated September 21, 2000, by and among
                        Mack-Cali Realty Corporation, Mack-Cali Realty, L.P.,
                        Prentiss Properties Trust, Prentiss Properties Acquisition
                        Partners, L.P. and Chicago Title Insurance Company, as
                        escrow agent.

         10.3           Purchase and Sale Contract, dated September 21, 2000, by and
                        between Mack-Cali Texas Property L.P. and Prentiss
                        Properties Acquisition Partners, L.P.

         99.1           News Releases dated September 22, 2000.
</TABLE>

                                       4