<PAGE>
                                                                    Exhibit 10.5

                                 PROMISSORY NOTE
                                 ---------------

$100,000.00
13 April 2001
--
Fairfax, Virginia

FOR VALUE RECEIVED, the undersigned George J. Pedersen (the "Maker") an
individual having an address at 700 Potomac Knolls Drive, McLean, VA 22101,
promises to pay to the order of ManTech International Corporation (the
"Holder"), the sum of One Hundred Thousand Dollars ($100,000.00) together with
interest at the rate of Eight Percent (8%) per year in equal monthly
installments of $667.67 commencing on May 11, 2001 and continuing on the same
day of each succeeding month thereafter until paid in full.

This Note may be prepaid at any time or from time to time in whole or in part
without penalty, premium or permission.

In the event of any default hereof, all remaining payments shall become due and
payable at the option of the Holder and the Maker agrees to pay all costs of
collection, including reasonable attorneys' fees if placed in the hands of an
attorney for collection. Upon default, the Maker further authorizes any Clerk of
any Court of Record in the Commonwealth of Virginia or elsewhere to enter
judgment by confession against them in favor of the Holder hereof for the full
amount appearing due and payable hereon, together with interest, charges,
reasonable attorneys' fees and costs of suit, as above provided. Demand or
presentment for payment, notice of dishonor, protest and notice of protest are
hereby waived.

In the event any one or more of the provisions contained in this Note shall for
any reason be held to be invalid, illegal or unenforceable in any respect, such
invalidity, illegality or unenforceability shall not affect any other provision
of this Note, but this Note shall be construed as if such invalid, illegal or
unenforceable provision had never been contained herein.

This Note may not be changed orally but only by an agreement in writing and
signed by the paities against whom enforcement of any waiver, change,
modification or discharge is sought

This Note shall be interpreted in accordance with the laws of the Commonwealth
of Virginia.


MAKER:

/s/ George J. Pedersen
-----------------------------
George J. Pedersen


WITNESS/ATTEST:

/s/ J. Jaye Free
------------------------------
J. Jaye Free

<PAGE>
                                                                    Exhibit 10.5


                                 PROMISSORY NOTE
                                 ---------------

$16,795.93                                                       June 12, 1996

     FOR VALUE RECEIVED, George J. Pedersen, 700 Potomac Knolls Drive, McLean,
VA 22101, promises to pay to order of ManTech International Corporation on
demand as the holder of this note or at such other place as the holder of this
note might from time to time designate in writing, the principal sum of Sixteen
Thousand Seven Hundred Ninety Five Dollars and 93/100 ($16,795.93), plus
interest on the principal balance. Interest at the rate of eight (8) percent
will be calculated on an annual basis and deducted from Pedersen paycheck each
pay period. At present ManTech's systems provides for 26 paychecks per year. The
interest rate may be adjusted from time to time by Phoenix Home Life Mutual
Insurance Company and Pedersen note will be adjusted accordingly.

     1. This note may be fully or partially prepaid at any time without penalty.

     2. If the undersigned defaults in the payment of any installment due under
this note and fails to cure such default within thirty (30) days after the
holder of this note has sent written notice of default, then, at the option of
the holder of this note, the entire principal balance of this note and all
accrued and unpaid interest shall at once become due. Failure to exercise this
option shall not constitute a waiver of the right to exercise the same in the
event of any subsequent default.

     3. The undersigned promises to pay all costs of collection, including
reasonable attorney's fees, upon default in the payment of the principal or
interest of this note when due, whether suit is brought or not.

     4. In the event any one or more of the provisions contained in this note is
held to be invalid, illegal or unenforceable for any reason, such invalidity,
illegality or unenforceability is not to affect any other provision of the note;
this note is to be construed as if such invalid, illegal or unenforceable
provision had never been a part of it.

     5. This note may not be changed orally but only by an agreement in writing
and signed by the party against whom agreement enforcement of any change is
sought.

     6. The note is to be interpreted in accordance with the laws of the
Commonwealth of Virginia.

ATTEST                                  NAME
/s/ J. Jaye Free                        /s/ George J. Pedersen
------------------------------          ---------------------------
                                        George J. Pedersen
                                        Chairman of the Board

<PAGE>
                                                                    Exhibit 10.5
FROM CORPORATE OFFICE

                                 PROMISSORY NOTE
                                 ---------------

$52,360.00                                                        23 June 1994


     FOR VALUE RECEIVED, George J. Pedersen, 700 Potomac Knolls Drive, McLean,
VA 22101, promises to pay to the order of ManTech International Corporation on
demand as the holder of this note or at such other place as the holder of this
note might from time to time designate in writing, the principal sum of Fifty
Two Thousand Three Hundred Sixty Dollars ($52,360.00), plus interest due on the
principal balance. Interest at the rate of eight (8) percent will be calculated
on an annual basis and deducted from Pedersen paycheck each pay period. At
present ManTech's systems provides for 26 paychecks per year. The interest rate
may be adjusted from time to time by Phoenix Home Life Mutual Insurance Company
and Pedersen note will be adjusted accordingly.

          1. This note may be fully or partially prepaid at any time without
penalty.

          2. If the undersigned defaults in the payment of any installment due
under this note and fails to cure such default within thirty (30) days after the
holder of this note has sent written notice of default, then, at the option of
the holder of this note, the entire principal balance of this note and all
accrued and unpaid interest shall at once become due. Failure to exercise this
option shall not constitute a waiver of the right to exercise the same in the
event of any subsequent default.

          3. The undersigned promises to pay all costs of collection, including
reasonable attorney's fees, upon default in the payment of the principal or
interest of this note when due, whether suit is brought or not.

          4. In the event any one or more of the provisions contained in this
note is held to be invalid, illegal or unenforceable for any reason, such
invalidity, illegality or unenforceability is not to affect any other provision
of the note; this note is to be construed as if such invalid, illegal or
unenforceable provision had never been a part of it.

          5. This note may not be changed orally but only by an agreement in
writing and signed by the party against whom agreement enforcement of any change
is sought.

          6. This note is to be interpreted in accordance with the laws of the
Commonwealth of Virginia.



                                       /s/ George J. Pedersen
                                       ---------------------------
                                       George J. Pedersen
                                       Chairman of the Board