Stewart belonged to a cadet branch of the Galloway family, owning estates in Wigtownshire and Kirkcudbright, extensive, but burdened with longstanding debts.
At the general election the Galloway family sponsored the candidature of Henry Watkin Dashwood in the burghs, but gave their interest to Stewart in Kirkcudbright for which he was returned after a contest. In the new Parliament every known vote of his was given on the Government side. No speech of his is recorded. He did not stand again in 1780, his finances being desperate. Disappointed in his hopes of a lucrative post, he had to be content with a minor Custom House place.
