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.