<PAGE>

                                                                   EXHIBIT 10.18

                             AMENDED AND RESTATED
                                PROMISSORY NOTE


Up to U.S.$150,000,000                                            April 27, 2001
                                                            Wilmington, Delaware

          FOR VALUE RECEIVED, the undersigned, MIRANT MID-ATLANTIC, LLC, a
Delaware limited liability company (the "Maker"), hereby unconditionally
promises to pay MIRANT AMERICAS GENERATION, INC., a Delaware corporation (the
"Holder"), at the offices of the Holder located at Suite 102, 1403 Foulk Road,
Wilmington, Delaware, 19803, or at such other place within the United States as
shall be designated from time to time by the Holder, on demand, the aggregate
unpaid principal amount of all sums advanced by the Holder to the Maker under
this Note (collectively, the "Loans"), not to exceed U.S.$150,000,000, as the
same may be set forth from time to time on the grid attached hereto as Schedule
A and made a part hereof, or such lesser principal amount as may then constitute
the aggregate unpaid balance of all Loans made by the Holder to the Maker
pursuant to this Note, in lawful money of the United States of America in
federal or other immediately available funds.

          The Maker also unconditionally promises to pay interest (calculated on
the basis of a year of 360 days and the actual number of days elapsed) on the
unpaid principal amount of the Loans outstanding from time to time for each day
from the date of disbursement until such principal amount is paid in full, at a
rate per annum equal to the Holder's total cost of borrowed funds from time to
time calculated by the Holder.  Interest calculated hereunder shall be due and
payable, semiannually, in arrears, on the first day of January and July of each
calendar year beginning on July 1, 2001.  Nothing contained in this Note shall
be deemed to establish or require the payment of a rate of interest in excess of
the maximum rate permitted by any applicable law.  In the event that any rate of
interest required to be paid hereunder exceeds the maximum rate permitted by
applicable law, such interest rate shall be reduced to the extent necessary to
comply with applicable law, and any interest amounts paid by Maker to Holder in
excess of such applicable rate shall be held in trust by the Holder for the
benefit of the Maker, to be remitted to Maker.

          The Holder shall endorse all borrowings made by the Maker under this
Note and all payments of principal of such borrowings on the grid attached
hereto as Schedule A and made a part hereof but no failure to make or any error
in making such endorsements shall affect the obligations of the Maker hereunder.

          The indebtedness evidenced by this Note shall be subordinated as
provided in Appendix A.

          Presentment for payment, demand, protest and notice of demand, notice
of dishonor, notice of non-payment and all other notices or demands in
connection with the delivery, acceptance, performance, default or endorsement of
this Note are hereby waived by the Maker.  No delay or omission on the part of
Holder in exercising any right hereunder shall operate as a waiver of such right
or any other right under this Note.  A waiver of any right or remedy on any one
occasion shall not be construed as a bar to or waiver of any right or remedy on
any future occasion.
<PAGE>

          The Maker shall be entitled, at any time and from time to time,
without the consent of Holder and without paying any penalty or premium
therefor, to prepay all or any portion or portions of the outstanding principal
amount of the Loans and accrued interest thereon.

          The Maker hereby agrees to pay on demand all costs and expenses
incurred in collecting the Loans and other obligations of the Maker hereunder or
in enforcing or attempting to enforce any of the Holder's rights hereunder,
including, but not limited to, reasonable attorneys' fees and expenses if
collected by or through an attorney, whether or not suit is filed.

          This Note shall be governed by, and construed in accordance with, the
laws of the State of Delaware without giving effect to the conflict of laws
principles thereof.


                                       MIRANT
                                       MID-ATLANTIC, LLC

                                       By: /s/ Gary J. Kubik
                                           -------------------------------------
                                           Name: Gary J. Kubik
                                                 -------------------------------
                                           Title: CFO and Treasurer
                                                  ------------------------------

Attest:

By: /s/ Michelle H. Ancosky
    ------------------------------
    Name: Michelle H. Ancosky
          ------------------------
    Title: Secretary
           -----------------------

<PAGE>

                                                                      Appendix A
                            Subordination Provisions
                            ------------------------

I.    The indebtedness evidenced by this Note and the other obligations
      described in the definition of "Subordinated Indebtedness" is subordinated
      and subject in right of payment to the prior payment in full of all
      obligations of the Facility Lessee under the Operative Documents (the
      "Senior Obligations"). Each holder of this Note, by its acceptance hereof,
      agrees to and shall be bound by all the provisions hereof. All terms used
      but not defined herein shall have the meaning set forth in Appendix A to
      the eleven Participation Agreements dated as of December 18, 2000, and
      listed on Appendix A-1.

II.   No payment on account of rent, principal, premium or interest on the
      Subordinated Indebtedness shall be made unless (a) full payment of all
      amounts then due and payable with respect to Senior Obligations has been
      made, (b) such payment would be permitted by the various Participation
      Agreements, (c) each of the conditions for the making of a "Restricted
      Payment" set forth in Section 6.8 of the various Participation Agreements
      has been satisfied and (d) immediately after giving effect to such
      payment, there shall not exist any Significant Lease Default, Lease Event
      of Default, or any event or occurrence, which, with passage of time or the
      giving of notice or both, would become a Lease Event of Default. Any such
      payment permitted pursuant to this paragraph is hereinafter referred to as
      a "Permitted Payment". For the purposes of these provisions, no Senior
      Obligation shall be deemed to have been paid in full until the obligee of
      such Senior Obligation shall have indefeasibly received payment in full in
      cash. Any cash payment on the Senior Obligations made by a Person which is
      solvent at the time of and after giving effect to such payment shall be
      presumed to have been indefeasibly paid in full in cash absent actual
      knowledge by the payor to the contrary. Any failure to make any payment on
      this Note prior to the time permitted by this paragraph shall not
      constitute a default under this Note.


III.  Upon any payment or distribution of assets of the Facility Lessee of any
      kind or character, whether in cash, property or securities, to creditors
      upon any dissolution or winding up or total or partial liquidation or
      reorganization of the Facility Lessee, whether voluntary or involuntary or
      in bankruptcy, insolvency, receivership or other proceedings, then and in
      any such event all Senior Obligations and all amounts (including interest,
      indemnities and fees) due or to become due upon all Senior Obligations
      shall first be paid in full before the holders of the Subordinated
      Indebtedness shall be entitled to retain any assets so paid or distributed
      in respect of the Subordinated Indebtedness (for principal, premium,
      interest or otherwise) and upon any such dissolution or winding up or
      liquidation or reorganization, any payment or distribution of assets of
      the Facility Lessee of any kind or character, whether in cash, property or
      securities, to which the holders of the Subordinated Indebtedness would be
      entitled, except as otherwise provided herein, shall be paid by the
      Facility Lessee or by any receiver, trustee in bankruptcy, liquidating

                                      A-3
<PAGE>

      trustee, agent or other person making such payment or distribution, or by
      the holders of the Subordinated Indebtedness if received by them, directly
      to the holders of the Senior Obligations or their representatives for
      application to such Senior Obligations in accordance with their terms. So
      long as any Senior Obligation is outstanding, the holder of this Note
      shall not commence, or join with any creditor other than any Person to
      whom Senior Obligations are owed, in commencing, or directly or indirectly
      causing the Facility Lessee to commence, or assist the Facility Lessee in
      commencing, any proceeding referred to in the preceding sentence.

IV.   The holder of this Note hereby irrevocably authorizes and empowers
      (without imposing any obligation on) each Person to whom any Senior
      Obligation is owed and such Person's representatives, under the
      circumstances set forth in the immediately preceding paragraph, to demand,
      sue for, collect and receive every such payment or distribution described
      therein and give acquittance therefor, to file claims and proofs of claims
      in any statutory or nonstatutory proceeding, to vote such Person's ratable
      share of the full amount of the Subordinated Indebtedness evidenced by
      this Note in its sole discretion in connection with any resolution,
      arrangement, plan of reorganization, compromise, settlement or extension
      and to take all such other action (including the right to participate in
      any composition of creditors and the right to vote such Person's ratable
      share of the Subordinated Indebtedness, evidenced by this Note, at
      creditors' meetings for the election of trustees, acceptances of plans and
      otherwise), in the name of the holder of the Subordinated Indebtedness
      evidenced by this Note or otherwise, as such Person's representatives may
      deem necessary or desirable for the enforcement of the subordination
      provisions of this Note. The holder of this Note shall execute and deliver
      to each Person to whom any Senior Obligation is owed and such Person's
      representatives all such further instruments confirming the foregoing
      authorization, and all such powers of attorney, proofs of claim,
      assignments of claim and other instruments, and shall take all such other
      action as may be reasonably requested by such Person or such Person's
      representatives in order to enable such holder to enforce all claims upon
      or in respect of such Person's ratable share of the Subordinated
      Indebtedness evidenced by this Note; provided, however, that nothing
      herein shall obligate the holder of this Note to pay or incur any cost,
      expense or liability or to take any action that may result in the
      imposition of any cost, expense or liability.

V.    The holder of this Note shall not, without the prior written consent of
      the various Owner Lessors and, so long as the Lien of the various Lease
      Indentures shall not have been discharged or terminated, the various Lease
      Indenture Trustees, have any right to accelerate payment of, or institute
      any proceedings to enforce, the Subordinated Indebtedness so long as any
      Senior Obligation is outstanding.

VI.   If any payment (other than a Permitted Payment) or distribution of assets
      of the Facility Lessee of any kind or character, whether in cash, property
      or securities, shall be received by the holder of the Subordinated
      Indebtedness before all Senior Obligations are paid in full, such payment
      or distribution will be held in trust for the benefit of, and shall be

                                      A-4
<PAGE>

      immediately paid over to, the holders of the Senior Obligations or their
      representatives for application to such Senior Obligations in accordance
      with their terms.

VII.  Nothing contained in this Note is intended to or shall impair as between
      the Facility Lessee, its creditors (other than the holders of Senior
      Obligations) and the holders of the Subordinated Indebtedness, the
      obligations of the Facility Lessee, to pay to the holders of the
      Subordinated Indebtedness, as and when the same shall become due and
      payable in accordance with their terms, or to affect the relative rights
      of the holders of the Subordinated Indebtedness and creditors of the
      Facility Lessee (other than holders of Senior Obligations).

VIII. The Persons to whom Senior Obligations are due shall not be prejudiced in
      their rights to enforce the subordination contained herein in accordance
      with the terms hereof by any act or failure to act on the part of the
      Facility Lessee.

IX.   The holder of this Note agrees to execute and deliver such further
      documents and to do such other acts and things as the Lease Financing
      Parties may reasonably request in order fully to effect the purposes of
      these subordination provisions. Each holder of this Note by its acceptance
      hereof authorizes and directs the Lease Financing Parties on its behalf to
      take such further action as may be necessary to effectuate the
      subordination as provided herein and appoints the various Lease Indenture
      Trustees, so long as the Lien of the various Lease Indentures shall not
      have been terminated or discharged in accordance with the terms thereof
      and thereafter the various Owner Lessors, as its attorney-in-fact for any
      and all such purposes.

X.    The subordination effected by these provisions, and the rights of the
      Persons to whom Senior Obligations are owed, shall not be affected by (i)
      any amendment of, or addition or supplement to any Operative Document, or
      any other document evidencing or securing any Senior Obligation, (ii) any
      exercise or nonexercise of any right, power or remedy under or in respect
      of any Operative Document, or any other document evidencing or securing
      any Senior Obligation or (iii) any waiver, consent, release, indulgence,
      extension, renewal, modification, delay, or other action, inaction or
      omission, in respect of, any Operative Document, or any other document
      evidencing or securing any Senior Obligation, whether or not any holder of
      any Subordinated Indebtedness shall have had notice or knowledge of any of
      the foregoing. No failure on the part of any holder of a Senior Obligation
      to exercise, and no delay in exercising, any right hereunder shall operate
      as a waiver thereof, nor shall any single or partial exercise of any right
      hereunder preclude any other or further exercise thereof or the exercise
      of any other right. The remedies herein provided are cumulative and not
      exclusive of any remedies provided by law.


XI.   The holder of this Note and the Facility Lessee each hereby waive
      promptness, diligence, notice of acceptance and any other notice with
      respect to any Senior Obligation and these

                                      A-5
<PAGE>

      terms of subordination and any requirement that any Lease Financing Party
      protect, secure, perfect or insure any Lien or any property subject
      thereto or exhaust any right to take any action against the Facility
      Lessee or any other Person or any collateral.

XII.  These terms of subordination shall continue to be effective or be
      reinstated, as the case may be, if at any time any payment of any Senior
      Obligation is rescinded or must otherwise be returned by the Person to
      whom such Senior Obligation is owed upon the insolvency, bankruptcy or
      reorganization of the Facility Lessee or otherwise, all as though such
      payment had not been made.

XIII. The provisions of these terms of subordination constitute a continuing
      agreement and shall (i) remain in full force and effect until each of the
      Facility Leases has been terminated and all amounts owed thereunder have
      been paid and the Lien of the various Lease Indentures has been terminated
      or discharged, (ii) be binding upon the holder of this Note and the
      Facility Lessee and their respective successors, transferees and assignees
      and (iii) inure to the benefit of, and be enforceable by, the Lease
      Financing Parties. Without limiting the generality of the foregoing clause
      (iii), the Lease Financing Parties may assign or otherwise transfer all or
      any portion of their rights and obligations under all or any of the
      Operative Documents to any other Person (to the extent permitted by the
      Operative Documents), and such other Person shall thereupon become vested
      with all the rights in respect thereof granted to the Lease Financing
      Parties herein or otherwise.


XIV.  This Note shall be governed by and construed in accordance with, the laws
      of the State of New York but without giving effect to the conflict of laws
      principles thereof which would require the application of the law of any
      other jurisdiction.

                                      A-6
<PAGE>

                                                                    Appendix A-1

                            Participation Agreements
                            ------------------------

1.  Participation Agreement (Dickerson L1) among Mirant Mid-Atlantic, LLC, as
Lessee, Dickerson OL1 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager, SEMA OP3 LLC, as Owner Participant, and State Street Bank and Trust
Company of Connecticut, National Association, as Lease Indenture Trustee and as
Pass through Trustee.

2.  Participation Agreement (Dickerson L2) among Mirant Mid-Atlantic, LLC, as
Lessee, Dickerson OL2 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager, SEMA OP6 LLC, as Owner Participant, and State Street Bank and Trust
Company of Connecticut, National Association, as Lease Indenture Trustee and as
Pass through Trustee.

3.  Participation Agreement (Dickerson L3) among Mirant Mid-Atlantic, LLC, as
Lessee, Dickerson OL3 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager, SEMA OP7 LLC, as Owner Participant, and State Street Bank and Trust
Company of Connecticut, National Association, as Lease Indenture Trustee and as
Pass through Trustee.

4.  Participation Agreement (Dickerson L4) among Mirant Mid-Atlantic, LLC, as
Lessee, Dickerson OL4 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager, SEMA OP8 LLC, as Owner Participant, and State Street Bank and Trust
Company of Connecticut, National Association, as Lease Indenture Trustee and as
Pass through Trustee.

5.  Participation Agreement (Morgantown L1) among Mirant Mid-Atlantic, LLC, as
Lessee, Morgantown OL1 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager, SEMA OP1 LLC, as Owner Participant, and State Street Bank and Trust
Company of Connecticut, National Association, as Lease Indenture Trustee and as
Pass Through Trustee, dated as of December 18, 2000.

6.  Participation Agreement (Morgantown L2) among Mirant Mid-Atlantic, LLC, as
Lessee, Morgantown OL2 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager, SEMA OP2 LLC, as Owner Participant, and State Street Bank and Trust
Company of Connecticut, National Association, as Lease Indenture Trustee and as
Pass Through Trustee, dated as of December 18, 2000.

7.  Participation Agreement (Morgantown L3) among Mirant Mid-Atlantic, LLC, as
Lessee, Morgantown OL3 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager, SEMA OP4 LLC, as Owner Participant, and State Street Bank and Trust
Company of Connecticut, National Association, as Lease Indenture Trustee and as
Pass Through Trustee, dated as of December 18, 2000.

8.  Participation Agreement (Morgantown L4) among Mirant Mid-Atlantic, LLC, as
Lessee, Morgantown OL4 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager,


                                     A-1-1
<PAGE>

SEMA OP5 LLC, as Owner Participant, and State Street Bank and Trust Company of
Connecticut, National Association, as Lease Indenture Trustee and as Pass
Through Trustee, dated as of December 18, 2000.

9.  Participation Agreement (Morgantown L5) among Mirant Mid-Atlantic, LLC, as
Lessee, Morgantown OL5 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager, SEMA OP8 LLC, as Owner Participant, and State Street Bank and Trust
Company of Connecticut, National Association, as Lease Indenture Trustee and as
Pass Through Trustee, dated as of December 18, 2000.

10.  Participation Agreement (Morgantown L6) among Mirant Mid-Atlantic, LLC, as
Lessee, Morgantown OL6 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager, SEMA OP9 LLC, as Owner Participant, and State Street Bank and Trust
Company of Connecticut, National Association, as Lease Indenture Trustee and as
Pass Through Trustee, dated as of December 18, 2000.

11.  Participation Agreement (Morgantown L7) among Mirant Mid-Atlantic, LLC, as
Lessee, Morgantown OL7 LLC, as Owner Lessor, Wilmington Trust Company, as Owner
Manager, SEMA OP9 LLC, as Owner Participant, and State Street Bank and Trust
Company of Connecticut, National Association, as Lease Indenture Trustee and as
Pass Through Trustee, dated as of December 18, 2000.


                                     A-1-2