A Democrat, he was elected Governor of Missouri in 1856 and served from January 5, 1857, until February 27 when he resigned to become a U.S.

Hancock Lee Jackson succeeded him as governor until the election of Robert Marcellus Stewart.

Polk was expelled from the U.S. Senate January 10, 1862, for his support of the South in the American Civil War.

He was appointed as a colonel in the Missouri State Guard under the command of Confederate General Sterling Price, and later served as a judge in the military courts of the Department of Mississippi in 1864 and 1865.

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