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 Horsehoe Ridge. At the Battle of Missionary Ridge near Chattanooga, Tennessee, Mitchell supported Maj. Gen. William T. Sherman's assault column. Later he moved to Ambrose Burnside's relief during the Siege of Knoxville. During the 1864 Atlanta Campaign, Mitchell led the advance on the enemy positions in the Battle of Rocky Face Ridge. His regiment suffered severely at the Battle of Resaca and played a prominent role in the battles of Dallas and New Hope Church. His old 113th lost over a hundred men in the assault on Kennesaw Mountain. 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.