[6] The firm was soon hit by the new revenue measures created to help support the finances of the Kingdom of Great Britain, including the much-reviled Stamp Act of 1765.

Under this role, he helped organize the strategic resources of Pennsylvania and later provided supplies, ships, and money in support of Pennsylvanian and French forces.

Although not a leading member of that convention, he supported a strong national government, the end of slavery, the United States Congress's powers to impose a tariff on imports and exports, the granting of the House of Representatives, and power in equal to the United States Senate in making treaties.

[9] Fitzsimons failed to win re-election in 1794, being defeated by John Swanwick, who carried seven of Philadelphia's twelve districts with 57% of the vote.

Although he never held elective office again, Fitzsimons served in 1798 as head of the committee of merchants overseeing the subscription loan to build a warship at private expense for use in the Quasi-War.

[10] FitzSimons, Innes, and, Samuel Sitgreaves, who replaced Innes upon the latter's death, became annoyed with the arguments used by their three British counterparts, Thomas Macdonald, Henry Pye Rich, and John Guillemard, to inflate the claims total, and FitzSimons and Sitgreaves angrily and permanently seceded from the board in July 1799.

[11] The claims were eventually disposed of by a lump-sum payment, agreed upon by United States Minister to Britain Rufus King with British Foreign Secretary Robert Banks Jenkinson and approved by President Thomas Jefferson and the Senate in 1802.

[5][13] 1791–1793 alongside: Frederick A.C. Muhlenberg, Thomas Hartley, Israel Jacobs, John W. Kittera, Daniel Hiester, William Findley, and Andrew Gregg