He was practicing law in Warm Springs, Virginia when the American Civil War began. He joined the Confederate Army and was elected major of the 13th Virginia Infantry Regiment in May 1861. He became colonel of the regiment after the Battle of Chancellorsville on May 15, 1863. On May 30, 1864, he was killed in action at the Battle of Totopotomoy Creek or Battle of Bethesda Church, which immediately preceded the Battle of Cold Harbor during the Overland Campaign. Terrill had already been nominated to the grade of brigadier general. The Confederate Senate posthumously confirmed the appointment on May 31, 1864, to rank from June 1, 1864.