Johnston was first commissioned into the
Royal Horse Guards and was present with the
regiment at the
Battle of Dettingen in 1743 and the
Battle of Fontenoy in 1745. He became a major in the regiment on 29 November 1750, was promoted to lieutenant-colonel of the Royal Horse Guards on 17 December 1754 and commanded the regiment in Germany in several engagements during the
Seven Years' War. Desperately wounded at the
battle of Minden in
1759 he was promoted to full
colonel of Horse, Royal Horse Guards 9 March 1762, (the same day as his 'Irish' cousin was gazetted colonel of Dragoons) and appointed an
aide-de-camp to
King George III. Promoted to major-general on 30 April 1770 on 2 May 1775 he received the additional appointment of colonel of the
11th Regiment of Dragoons in place of the marquess of Lothian and was further promoted to lieutenant-general on 29 August 1777. On 2 April 1778 he was removed from his colonelcy of Royal Horse Guards and a fortnight later appointed colonel of the
6th Regiment of Dragoons. In succession to
George Preston, his sister's husband, he was made colonel of the
Scots Greys on 4 February 1785, a post he held until his death. ==Personal life==