In 1850 he was assigned to duty on the frontier, serving with the 8th Infantry, but he was recalled and detailed as Assistant Professor of Mathematics at West Point until 1856.

[1] Cogswell then rejoined the 8th Infantry in New Mexico, participating in a skirmish with the Navajo on October 6, 1858 and remaining on station until 1860.

In July 1861, he was made Colonel of the Forty-Second New York Volunteers, and at the Battle of Ball's Bluff was captured by the Confederates and incarcerated in Libby Prison, until being exchanged.

At the close of the war he was assigned to garrison duty at Baltimore, and afterward served as Acting Judge-Advocate of the Department of North Carolina.

[4] A distant relation, William S. Cogswell Jr., was elected Mayor of Charleston, taking office in January 2024.