 Mitchell was promoted lieutenant in 1947, captain in 1952, major in 1959, and his success in a wide range of appointments won him brevet rank as a lieutenant-colonel in 1964.Mitchell was promoted to substantive lieutenant-colonel on 31 December 1966, and made Commanding Officer 1st Battalion, The Argyll and Sutherland Highlanders (the ‘Argylls’) on 12 January 1967.In 1967 the British Aden colony consisted of the City of Aden and attached Protectorate areas, with a total land mass similar to that of the United Kingdom.