King's knowledge, bearing, and oratorical gifts soon launched him on a political career. From 1783 to 1785 he was a member of the Massachusetts legislature, after which that body sent him to the Continental Congress (1784-86). There, he gained a reputation as a brilliant speaker and an early opponent of slavery. Toward the end of his tour, in 1786, he married Mary Alsop, daughter of a rich New York City merchant. He performed his final duties for Massachusetts by representing her at the Constitutional Convention and by serving in the commonwealth's ratifying convention.

At age 32, King was not only one of the most youthful of the delegates at Philadelphia, but was also one of the most important. He numbered among the most capable orators. Furthermore, he attended every session. Although he came to the convention unconvinced that major changes should be made in the Articles of Confederation, his views underwent a startling transformation during the debates. With Madison, he became a leading figure in the nationalist caucus. He served with distinction on the Committee on Postponed Matters and the Committee of Style. He also took notes on the proceedings, which have been valuable to historians.

About 1788 King abandoned his law practice, moved from the Bay State to Gotham, and entered the New York political forum. He was elected to the legislature (1789-90), and in the former year was picked as one of the state's first U.S. senators. As political divisions grew in the new government, King expressed ardent sympathies for the Federalists. In Congress, he supported Hamilton's fiscal program and stood among the leading proponents of the unpopular Jay's Treaty (1794).

Meantime, in 1791, King had become one of the directors of the First Bank of the United States. Reelected to the U.S. Senate in 1795, he served only a year before he was appointed as Minister to Great Britain (1796-1803).

King's years in this post were difficult ones in Anglo-American relations. The wars of the French Revolution endangered U.S. commerce in the maritime clashes between the French and the British. The latter in particular violated American rights on the high seas, especially by the impressment of sailors. Although King was unable to bring about a change in this policy, he smoothed relations between the two nations.

In 1803 King sailed back to the United States and to a career in politics. In 1804 and 1808 fellow-signer Charles Cotesworth Pinckney and he were the Federalist candidates for President and Vice President, respectively, but were decisively defeated. Otherwise, King largely contented himself with agricultural pursuits at King Manor, a Long Island estate he had purchased in 1805. During the War of 1812, he was again elected to the U.S. Senate (1813-25) and ranked as a leading critic of the war. Only after the British attacked Washington in 1814 did he come to believe that the United States was fighting a defensive action and to lend his support to the war effort.

In 1816 the Federalists chose King as their candidate for the presidency, but James Monroe beat him handily. Still in the Senate, that same year King led the opposition to the establishment of the Second Bank of the United States. Four years later, believing that the issue of slavery could not be compromised but must be settled once and for all by the immediate establishment of a system of compensated emancipation and colonization, he denounced the Missouri Compromise.

In 1825, suffering from ill health, King retired from the Senate. President John Quincy Adams, however, persuaded him to accept another assignment as Minister to Great Britain. He arrived in England that same year, but soon fell ill and was forced to return home the following year. Within a year, at the age of 72, in 1827, he died. Surviving him were several offspring, some of whom also gained distinction. He was laid to rest near King Manor in the cemetery of Grace Episcopal Church, Jamaica, Long Island, NY.

Biographical information adapted from the National Archives.