Franklin Pierce entered the New Hampshire House of Representatives in 1929, served as its speaker in 1931 and 1832, spent two terms in the U.S. House of Representatives, and then moved to the U.S. Senate in 1837. He resigned his Senate seat in 1842 and returned to New Hampshire, where he served as a Democratic political manager.

With the outbreak of the Mexican War, Pierce volunteered and was commissioned a colonel and then a brigadier general. He joined Gen. Winfield Scott in his campaign against Mexico City, but an injury kept him from further action. Returning to civilian life, his support of the Compromise of 1850 and the Fugitive Slave Laws won him support in the South.

In the Democratic convention of 1852, he won the nomination on the 49th ballot. Pressured by some influential senators, he endorsed the Kansas-Nebraska Bill, which dealt with slavery in the new territories and left the decision to the settlers themselves. The measure was opposed strenuously in the North, and, following its passage, the Republic Party was formed. Civil war erupted in the Kansas Territory in 1856, and Pierce recognized the pro-slavery government. He sent troops to the territory.

The Gadsden Purchase was made during Pierce's administration, and a treaty that obtained fishing rights for Americans off Newfoundland was signed with Great Britain. Pierce retired from public life after leaving the presidency.