Seth Thornton was commissioned as a lieutenant in the 2nd Dragoon Regiment in June 1836, and was promoted to captain in February 1841.
On 25 April 1846, while leading 2 companies of his regiment along the Rio Grande, Thornton and his troops were ambushed by a large Mexican force. This skirmish became known as the "Thornton Affair", and was the opening engagement of the Mexican-American War.
Thornton was wounded and captured during the battle, and exchanged a few weeks later. He was eventually killed by cannon fire at San Antonio during the advance towards Mexico City.