He won election to represent New York in the United States Senate in 1789, remaining in office until 1796. That year, he accepted President George Washington's appointment to the position of Minister to Britain. Though King aligned with Hamilton's Federalists, Democratic-Republican President Thomas Jefferson retained his services after Jefferson's victory in the 1800 presidential election. King served as the Federalist vice presidential candidate in the 1804 and 1808 elections, running on an unsuccessful ticket with Charles Cotesworth Pinckney of South Carolina. Though most Federalists supported Democratic-Republican DeWitt Clinton in the 1812 presidential election, King, without the support of his party, won few votes of those Federalists who were unwilling to support Clinton's candidacy. In 1813, he returned to the Senate and remained in office until 1825.