After the American Civil War broke out, Mitchell enlisted in Company C of the 13th Iowa Infantry Regiment.
Mitchell saw action at the Siege of Vicksburg, during the Meridian campaign, and was captured at the Battle of Atlanta.
Mitchell's release was secured near the end of the war, and he was mustered out of service with the 18th Iowa on July 21, 1865.
After winning reelection to the state house in 1893, Mitchell won the District 6 seat in the 1895 Iowa Senate election.
Around 1927, Mitchell relocated to New York City to improve his wife's health and be nearer to one of his daughters.