After failing to win reelection to the Congress Morris moved to Philadelphia and resumed his law practice. A series of newspaper articles on finance secured him the post of assistant to Robert Morris (no relative) in handling the finances of the new government (1781-85). In this position he planned the U.S. decimal coinage system. As a member of the U.S. Constitutional Convention of 1787 Morris played an active role, defending a strong centralized government and a powerful executive, opposing concessions on slavery, and putting the Constitution into its final literary form. He remained, however, a champion of aristocracy who distrusted democratic rule.
In 1789 Moris went to France as a private business agent, remained in Europe, and was appointed (1792) U.S. minister to France. During the French Revolution his sympathies lay with the royalists; he even helped plan a scheme to rescue Louis XVI. His recall was requested in 1794, but he traveled for several years before returning to America in 1798. From 1800 to 1803, Morris, a Federalist, was a U.S. senator from New York. He then retired to his estate. He condemned the War of 1812, going so far as to recommend the severance of the federal union. Morris was a strong advocate of the Erie Canal and served as chairman (1810-13) of the canal commission.
See his Diary of the French Revolution (1939), edited by his great-granddaughter, Beatrix Cary Davenport; biographies by T. Roosevelt (1888, repr. 1972), D. Walther (tr. 1934), and R. Brookhiser (2003); M. M. Mintz, Gouverneur Morris and the American Revolution (1970).