Battle of Dettingen
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.