Elliott was admitted to the bar shortly before the outbreak of hostilities, whereupon he enlisted in the 5th Ohio Infantry. In 1862, the regiment was sent into Virginia's Shenandoah Valley. On June 9. 1862, he was captured at the Battle of Port Republic but was exchanged three months later and returned to his regiment. Elliot saw action at Chancellorsville, Gettysburg, Lookout Mountain and took part in the capture of Atlanta. After the death in combat of two of his brothers, he acceded to his mother's request and left the army some six months after his enlistment expired.