Cass served as a brigadier general in the War of 1812. He served as governor of the Territory of Michigan from 1813 to 1831, then he became United States Secretary of War for President Andrew Jackson from 1831 to 1836, and then as ambassador to France from 1836 to 1842. Cass represented Michigan in the United States Senate from 1845 to 1848. He resigned from the Senate to run for President, then returned to the Senate after losing the election, and served from 1849 to 1857. Cass served as President James Buchanan's United States Secretary of State from 1857 to 1860.
Cass was the United States Democratic Party candidate for president in 1848, but he lost the election to Zachary Taylor. Cass was a leading supporter of the Doctrine of Popular Sovereignty, which held that the people who lived in a territory should decide whether or not to permit slavery there. His nomination caused a split in the Democratic party, leading many antislavery Democrats to join the Free Soil Party.