Enlisted in the Union Army in 1861. On 5 May 1861 he was assigned to Company K, 26th Pennsylvania Regiment (Volunteer) for approximately three years. The regiment did see significant action during the Civil War. He was discharged on 3 Jan 1864. He then enlisted in Company H, 99th Pennsylvania Veteran Volunteer Regiment on 23 Dec 1865, near Washington D.C. He was a waterman and was 5 feet 7 3/4 inches tall, blue eyes, dark hair. Thomas was on leave in Washington one evening a few months before his discharge and according to family lore (tradition), he was actually in attendance at Ford's Theatre when Lincoln's assassination took place.
Source: A History of the Carlson Family and Related Families, Hugh Robert Carlon, 1967.