John Quincy Adams began a diplomatic career in the 1790s, serving as minister to the Netherlands and Prussia. After returning to Massachusetts politics he entered the United States Senate in 1803. His support for measures of Thomas Jefferson's administration, including the embargo, estranged Federalist allies. James Madison appointed him minister to Russia in 1809. He helped negotiate the Treaty of Ghent ending the War of 1812 and then represented the United States in Britain.

As secretary of state under James Monroe from 1817 to 1825, Adams played a central role in defining the country's borders and foreign policy. He negotiated the Adams-Onís Treaty with Spain, securing Florida and setting a transcontinental boundary, and helped formulate the Monroe Doctrine's warning against new European colonial intervention. He favored national independence in foreign affairs and resisted making the United States subordinate to a British proposal for a joint declaration.

The presidential election of 1824 produced no electoral majority. The House selected Adams in 1825, and his appointment of Henry Clay as secretary of state fueled opponents' claims of a corrupt bargain, although a secret agreement has not been established. Adams advocated roads, canals, scientific and educational institutions, and other national improvements. Congressional resistance and an increasingly organized opposition limited his program. Andrew Jackson defeated him in the bitter election of 1828, ending his single presidential term.

Adams returned to public office as a member of the House of Representatives in 1831, an unusual second career for a former president. He defended the right to petition and repeatedly challenged the gag rules that blocked consideration of antislavery petitions. Although he had long held complicated views about slavery and federal power, he became an increasingly forceful opponent of its political influence. In 1841 he argued before the Supreme Court on behalf of the Africans who had resisted captivity aboard the Amistad, supporting their freedom. He continued serving in the House until collapsing there shortly before his death in 1848. His extensive diary documents decades of political and personal life. His career joins major diplomatic achievements, an obstructed presidency, and a later congressional defense of civil liberties and opposition to slavery that substantially changed his historical reputation.