Unlike his older brothers, Sir Michael, Admiral Sir Houston Stewart (1791-1875) and the advocate John Shaw Stewart (1793-1840), little can be said with certainty about Stewart’s early life. He joined the London firm of P. Stewart trading at Winchester House, Great Winchester Street before October 1812, when, writing as a merchant, he advised his father, who had recently succeeded to the baronetcy and estates of Sir John Shaw Stewart of Blackhall and Ardgowan, so becoming the largest landowner in the Clydeside port of Greenock, to switch from the Baltic to the West India trade and develop the family’s Houston and Roxborough plantations in Tobago.
A bold public speaker and persuasive negotiator, with a reputation for conviviality and wit, he was instrumental in securing support for Sir Michael when he successfully contested Lanarkshire at the 1827 by-election, and helped to effect his switch to Renfrewshire at the general election of 1830, when they failed in their attempt to bring in their kinsman John Maxwell* for Lanarkshire.
Brought in to strengthen the West India lobby, promote reform, and attend to Scottish and commercial issues affecting his relations and their political allies, Stewart was admitted to Brooks’s on Lord Lansdowne’s nomination, 27 Mar., and attended West India committee strategy meetings on 28 Mar. and 13 Apr. 1831.
He voted for the second reading of the reintroduced reform bill, 6 July 1831, and gave it steady support in committee, taking care to inform the press if his name was omitted from the division lists.
Stewart was added to the planters and merchants’ subcommittee ‘for managing the question of the admission of molasses to use in breweries and distilleries’, 30 June,
Stewart was returned unopposed for Lancaster as a Liberal in 1832 and 1835, despite mounting concern over his stance on slavery and joint-stock enterprises. Defeated there in 1837, at the 1841 general election he recaptured the Renfrewshire seat lost to the Conservatives following his brother’s death in 1836.
