In the autumn of 1862, Mitchell helped raise and recruit what became the 113th Ohio Infantry and was appointed as the regiment's lieutenant colonel on September 2. On May 6, 1863, Mitchell continued his progression through the ranks, being elevated to colonel and leading his regiment during the Tullahoma Campaign.
On January 12, 1865, on the recommendation of General Sherman, the 26-year-old Mitchell was promoted to brigadier general of volunteers, becoming one of the youngest civilian-soldiers to attain that rank without benefit of preparative military training.