His family moved (1806) to the Duck River valley in Tennessee and there, after graduating from the Univ. of North Carolina (1818) and studying law under Felix Grundy, he began (1820) to practice law in Columbia. Polk served in the state legislature (1823–25) and in the U.S. House of Representatives (1825–39), where he was speaker for the years 1835–39. He was a leading Jacksonian Democrat. In 1839 he was elected governor of Tennessee, but he was defeated for reelection by the Whig candidate in 1841 and 1843.

Polk had vice presidential ambitions, but Andrew Jackson, convinced that Martin Van Buren had committed political suicide by announcing his opposition to the annexation of Texas, urged Polk to consider the presidency. With the Van Buren and Lewis Cass factions deadlocked at the Democratic convention at Baltimore in 1844, George Bancroft advanced Polk as a candidate behind whom both sections could unite, and the “dark horse” won the nomination. Polk campaigned on an expansionist platform and narrowly defeated Henry Clay by carrying New York state, where the presidential candidacy of James G. Birney of the Liberty party cut into Clay's vote.

To the surprise of many, the new President proved to be his own man; he even ignored Jackson's wishes on several matters. Renouncing a second term for himself, he required the members of his cabinet, which included James Buchanan, Robert J. Walker, William L. Marcy, and Bancroft, to devote all their energies to their offices, not to campaigning to succeed him.

Polk announced that his administration would achieve “four great measures”: reduction of the tariff; reestablishment of the independent treasury; settlement of the Oregon boundary dispute; and the acquisition of California. All were accomplished. The Walker Tariff, one of the lowest in U.S. history, was enacted in 1846, as was the bill restoring the Independent Treasury System. Despite the aggressive Democratic slogan “Fifty-four forty or fight,” the dispute with Great Britain over Oregon was peaceably resolved with the adoption of lat. 49°N (the 49th parallel) as Oregon's northern boundary.

Relations with Mexico, on the other hand, reached a breaking point after the annexation of Texas. Polk had hoped to purchase California and to settle other difficulties with Mexico by negotiation. However, after the failure of the mission of John Slidell to Mexico, the President ordered the American advance to the Rio Grande that precipitated the Mexican War. As a result of the war, the United States acquired not only California but the entire Southwest.

Few presidents have worked harder, and few have equaled Polk's record of attaining specific, stated aims. He labored so strenuously in fact that his health gave way, and he died a few months after leaving office.

• See The Diary of James K. Polk (ed. by Quaife, M. M. , 4 vol., 1910; abr. in 1 vol. by Nevins, A. , 1952);.
• his correspondence, ed. by H. Weaver and P. H. Bergeron (2 vol. 1969-72);.
• biographies by C. G. Sellers, Jr. (2 vol., 1957-66), C. A. McCoy (1960, repr. 1973), and W. R. Borneman (2008);.
• Merry, R. W. , A Country of Vast Designs: James K. Polk, the Mexican War, and the Conquest of the American Continent (2009).