Stewart’s father, a younger son of the 7th earl of Galloway, had a brief military career and sat for Wigtown Burghs on the family interest from 1806 until January 1809, when he was made a commissioner of the navy victualling board by the Portland ministry. He was subsequently, under the auspices of the Liverpool administration, paymaster of marines (1812-13), and a commissioner of the navy board (1813-19), of customs (1819-21) and of audit (1821-7). His marriage in 1805 connected him to the Charteris family, claimants to the attainted earldom of Wemyss. His brother-in-law Francis Charteris was created Baron Wemyss in 1821 and on the reversal of the attainder in 1826 became 6th earl of Wemyss.
Stewart was returned for Wigtown Burghs as a Liberal after a contest at the general election of 1832 and retired from Parliament at the dissolution in 1834. He was called to the bar that year but never practised, and made a change of direction by entering the church. He was ordained deacon in 1841, became vicar of Sparsholt, Hampshire in 1842 and in 1850 obtained the living of Lainton. On the death of his father in 1851 he received a legacy of £2,000 and a quarter share in residue of personal estate calculated for duty at £27,989.
