[1][2] Shortly after the outbreak of the Civil War in early 1861, Mitchell enlisted as a private in the first battalion of the Ohio Reserves.

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.

[1] During the Battle of Chickamauga, Mitchell led a brigade under James B. Steedman which arrived late on the second day and played a leading role in aiding George H. Thomas in holding onto his delaying position atop Horseshoe Ridge.

At the Battle of Missionary Ridge near Chattanooga, Tennessee, Mitchell supported Maj. Gen. William T. Sherman's assault column.

Mitchell reported to Thomas and took command of the detached XIV Corps during the Battle of Nashville and the subsequent pursuit of the retreating Confederate army of John Bell Hood.

[1] Mitchell resigned from the Army on July 3, 1865, and returned home to Columbus, Ohio, to resume his legal career.

Mitchell served as the Register in Bankruptcy, President of the Columbus City Council, and as the Ohio pension commissioner.