Garlies was the eldest son of John Stewart, 7th Earl of Galloway, and Anne, daughter of Sir James Dashwood, 2nd Baronet,[1] and attended Westminster School[2] before embarking on a career in the Royal Navy.

Garlies entered the navy at an early age, serving as a 13-year-old midshipman under the command of his uncle, Commodore Keith Stewart at the Battle of Dogger Bank in August 1781, and also in the Great Siege of Gibraltar in 1782.

He was promoted to post-captain on 30 April 1793, and soon after was appointed to the frigate Winchelsea, serving in the West Indies, and being wounded while covering the landing of the army at Guadaloupe in April 1794,[1][3] and was then sent with detachments of troops to accept the surrender of the islands of Marie-Galante and La Désirade.

Commanding a division of four frigates and a sloop, he engaged the Spanish ship of line San Francisco de Asís in the action of 25 January 1797, in which he was forced to withdraw.

He was first elected in 1790 for the constituency of Saltash, and served until vacating his seat in favour of his brother William in February 1795.