In 1777 he returned to Britain and was promoted to lieutenant, serving on the hired armed ship "Two Sisters" off Northern Scotland. In 1779 he joined the frigate HMS "Vestal", then under the command of Captain George Cranfield Berkeley, with whom Monkton was to have a lengthy and productive professional relationship. In this frigate, Monkton raided French shipping off Newfoundland and twice escorted convoys relieving Gibraltar during the Great Siege. "Vestal" also served at the destruction of a French convoy in the action of 20–21 April 1782.