When he was but sixteen, young Mitchell joined the army of the Peninsula as a volunteer.

Three years later he received a commission in the 95th Regiment or Rifle Brigade.

He was employed on the Quartermaster General's staff at military sketching; and he was present in the field at Ciudad Rodrigo, Badajoz, Salamanca, the Pyrenees, and St.Sebastian.After the close of the war he went to Spain and Portugal to survey the battlefields.

He received promotion to a Lieutenancy in 1813.

He served in the 2nd, 54th, and 97th Regiments of foot, and was promoted to be Captain in 1822, and Major in 1826.