He graduated from Yale College in 1826, and was a tutor at Yale College from 1828 to 1830. Then he studied law, was admitted to the bar in 1829, and commenced practice in Troy, New York.
Seymour was elected as a Democrat to the 28th United States Congress, holding office from March 4, 1843, to March 3, 1845, and was Chairman of the Committee on Revolutionary Pensions.
Seymour was elected to the 32nd United States Congress, holding office from March 4, 1851, to March 3, 1853, and was Chairman of the Committee on Commerce. Afterwards, he resumed the practice of law.