His father suffered severe financial losses in the late 1770s, and it was his uncle, General Alexander Gordon who encouraged him to join the army at the tender age of eleven.

[3] He saw action at the Battle of Salamanca, siege of Burgos, San Marcial, the Nivelle and Orthez.

He was mentioned in dispatches for San Marcial and was awarded a Gold Medal for commanding a battalion at Salamanca and Orthez.

[4] 'As the light was beginning to fail (on 17 June 1815) Colonel Mitchell led his brigade to the position ... on the extreme right near Braine-l'Alleud, and here the regiments bivouacked [sic] for the night, the rain continuing to fall in torrents.

Although the brigade did not hold a conspicuous part in the battle, they were vital to the defence of Wellington's right wing.

Mitchell was one of the few officers below the rank of general to be mentioned by the Duke of Wellington's Waterloo Dispatch.