Seymour entered politics early in life. In 1833, he moved to Albany, where he joined the staff of Governor William Marcy (1833-1839). Seymour made his electoral debut in 1841 when he won a seat in the New York state legislature, followed by a victorious run for the mayoralty of Utica in 1842. He returned to the legislature the following year and in 1845, having gained a reputation for effectively forging compromises between competing Democratic factions, was elected speaker of the assembly. Seymour became known for his tireless promotion of improvements to the Erie Canal. He was a member of the conservative "Hunker" wing of the New York Democracy, which favored state government support for internal improvements and backed the expansionist policies of President James K. Polk. The then-retired Seymour, however, did not play a major role in the dispute over the Mexican War and the expansion of slavery.

Seymour lost a bid for the New York governorship in 1850, but was elected two years later by a reunited Democratic party. As governor (1853-1855), Seymour oversaw the enactment of penal reform and opposed prohibition and nativism. In 1854, he was narrowly defeated for reelection in a four-way race which pitted him against candidates from the emerging Republican party, the nativist American party, and a splinter Democratic faction. Seymour retired to his farm, but worked behind the scenes in an attempt to keep the increasingly divided Democratic party united.

When his party did split in 1860, nominating two presidential candidates, Seymour backed Stephen Douglas and the policy of popular sovereignty for the Western territories (allowing the voters there to decide the question of slavery for themselves). After Republican Abraham Lincoln won the presidency, Seymour opposed secession and worked for a peaceful compromise. When the Civil War began, he supported the Union military effort and distanced himself from the antiwar Peace Democrats. Elected governor in 1862, Seymour worked hard to fill his state's military quotas. He was a vocal critic, however, of Lincoln administration policies, including government centralization of power, emancipation of the slaves, the military draft, and the suppression of civil liberties.

Governor Seymour managed to delay the implementation and limit the scope of the draft in New York, but its commencement in July 1863 provoked four days of violent riots in New York City (as it did in cities across the North). As governor, he traveled to the city, where he spoke to an angry crowd, addressing them controversially as "My friends." He was attempting to quell the rioters' violence, but Republicans would forever after tar him as sympathizing publicly with the perpetrators. In 1864, Seymour was defeated for reelection. Out of office, he continued to play an active and influential role in Democratic politics, supporting the lenient Reconstruction policies of President Andrew Johnson and opposing the alternative of the Republican Radicals.

After a lengthy deadlock at the Democratic National Convention in 1868, delegates chose Seymour was their compromise candidate for president. Despite his initial hesitation, Seymour ran a vigorous campaign, becoming only the second presidential nominee (after Stephen Douglas) to embark on an issues-oriented speaking tour. During the contest, Republicans associated the Democratic nominee with violence against blacks by reminding voters of his alleged complicity in the Civil War draft riots in New York City and by linking the him with Reconstruction violence. In the November election, Seymour lost to Republican Ulysses S. Grant, the military hero of the Civil War.

In his later years, Seymour was honored as an elder statesman of his party. He mentored younger Democrats, such as Samuel J. Tilden and Grover Cleveland, both of whom became governors of New York and presidential nominees of the Democratic party. Seymour lived long enough to see the latter elected president, but died in 1886 at his sister's house in Utica, New York.

Source consulted: American National Biography.