His father was a member of the diplomatic service who served as consul-general for Norway, and engaged on missions to the court of Stockholm and Copenhagen.
[1][2] He served with the same battalion in the Peninsula from 1810 to 1812, and was present at the battles of Busaco and Fuentes d'Onoro in the action of Sabugal, and in those of the retreat of Massena.
He accompanied the 4th battalion on the expedition under Major-general Samuel Gibbs to Stralsund in 1813, but served on the staff as a deputy assistant quartermaster-general.
[1][2] Mitchell did not return to military duty, but devoted himself to literature, passing a considerable portion of each year on the continent up to 1848, after which he spent the remainder of his life with his sisters in Edinburgh.
Between 1841 and 1855 he contributed to the United Service Journal, and in 1841 to 1842 he wrote seven letters to The Times dealing with defects in the British army.