A judge of
probate from 1836 to 1838, Seymour was also Editor of the
Jeffersonian from 1837 to 1838. In 1842, he was elected to the
U.S. House of Representatives and served one term from 1843 to 1845, declining reelection in 1844. During the
Mexican–American War, Seymour was commissioned as a major in the Connecticut Infantry on March 16, 1846, later recommissioned to the new
9th United States Infantry on April 9, 1847. Due to his courageous leadership at the
Battle of Chapultepec, he was promoted to
lieutenant colonel of the
12th Infantry under Colonel
Milledge L. Bonham on August 12, 1847. After the war, Seymour made an unsuccessful run for
Governor of Connecticut in
1849, but was elected governor by the
Connecticut General Assembly the next year in
1850. He was re-elected in
1851,
1852 and
1853. He served as an 1852
presidential elector, endorsing Franklin Pierce and, in return for his support, Seymour was appointed to serve as minister to Russia and resigned the governorship shortly after being reelected to a fourth term. He accepted the commission of
Minister to Russia from
President Franklin Pierce. He resigned from the governorship on October 13, 1853, and spent the next four years in Russia, where he built a warm and ongoing alliance with
tsar Nicholas and his son
tsar Alexander. He served in this position until 1858 when President
James Buchanan replaced him with
Francis W. Pickens. In Russia, his attaches included
Daniel Coit Gilman and
Andrew Dickson White. Seymour made two unsuccessful attempts to return to the governorship in
1860 and
1863 and unsuccessfully sought the
Democratic nomination for
President of the United States at the
1864 Democratic National Convention, losing to
Civil War general
George B. McClellan. ==Death and legacy==