Monroe was elected to the Virginia legislature in 1782 and served (1783-86) in the Continental Congress under the Articles of Confederation. He was not a delegate to the Constitutional Convention, and in his own state he supported Patrick Henry in opposing the Constitution, which seemed to him to create a government so centralized that it encroached on states' rights.

Under the new government, he served (1790-94) in the U.S. Senate, where he proved himself an outstanding lieutenant of Jefferson and a vigorous opponent of George Washington, Alexander Hamilton, and the Federalists. Appointed (1794) minister to France in the hope that his Francophile sympathies would smooth the ruffled relations between the two nations, he did nothing to lessen French resentment over Jay's Treaty, and he was recalled in 1796.

Governor of Virginia from 1799 to 1802, he was sent (1802) by President Jefferson to France as a special envoy. There he assisted Robert R. Livingston (1746-1813; see Livingston, family) during negotiations (1803) for the Louisiana Purchase. The next year, in Spain, he aided Charles Pinckney in the unsuccessful negotiations with the Spanish government. A later mission, to England, was even more disastrous. Monroe and William Pinkney struggled to arrive at a commercial treaty to end the disputes between Great Britain and the United States over shipping, but they could get no concessions, and Jefferson did not even submit the treaty they drafted (1806) to the Senate for approval.

In 1808, Monroe made a bid for the presidential nomination. He thus alienated James Madison, but the estrangement did not last long, and Monroe, after serving again as governor of Virginia, was Madison's Secretary of State (1811-17). For a time he was also Secretary of War (1814-15), after the dismissal of John Armstrong.

In 1816 Monroe obtained the presidential nomination and was easily elected. During his first administration, serious differences over the question of slavery in the territories were accommodated by the Missouri Compromise, which Monroe signed despite his sympathy for the South in this matter. In foreign affairs a number of settlements were reached. The Rush-Bagot agreement with Great Britain (1817) provided for mutual limitation of armaments on the Great Lakes, and the U.S.-Canadian boundary question was also settled. U.S. possession of the Floridas was confirmed by Andrew Jackson's campaigns and a treaty with Spain (1819).

In the 1820 election, despite economic depression, Monroe lost only one vote in the electoral college that reelected him. Late in 1823, he issued what came to be known as the Monroe Doctrine, one of the most important principles of U.S. foreign policy. Although this declaration was as much the work of Monroe's Secretary of State, John Quincy Adams, as of the President himself, the initiative for presenting it in the annual message to Congress was Monroe's. The experiment of the American Colonization Society in settling Liberia was undertaken with Monroe's blessing, and Monrovia was named for him.

At the end of his term Monroe retired to his estate, Oak Hill, near Leesburg, Va. In 1829 he presided over the Virginia constitutional convention and supported the conservatives on suffrage and slavery. He died during a visit to New York City.