Son of Sir Michael Stewart (ca. 1712-1796), 3rd of Blackhall, and Helen (or Helenora) Houstoun (d. 1746). Brother of Houston Stewart.
From ca. 1762 he was a merchant in Amsterdam. In 1770 he bought a plantation near Queen's Bay on Tobago, to which he added further lands in 1773. The combined lots became the Roxborough Estate, which he released to his brother Michael in 1775, and which was to become one of the largest sugar plantations on the island. He was killed in 1779, while attempting to protect his lands from an attack by American privateers.
When Boswell arrived in Holland in August 1763, he went immediately to see Stewart, to whom he had apparently been referred by Norton Nicholls.1 They didn't know each other well in advance, but, as Boswell wrote to Johnston some time later, [he] was met with every civility. A few days later Boswell left for Utrecht, leaving some of his luggage at Stewart's until he got himself settled. On the night between the 15th and 16th of August, Boswell returned from Utrecht to Rotterdam, feeling most depressed. He again turned to Stewart who “was very kind, took me to his house, talked with me, endeavoured to amuse me, and contrived schemes for me to follow.”2
Stewart tried to keep Boswell's spirits up, even after Boswell had settled in Utrecht, and in a letter of September 7, 1763, wrote “For recreation read a chapter now and then of the Great Man or honest Spec. Your tongue and p---k are the only two members I have not instructed you how to exercise. The former of these you must satisfy by half an hour's vociferation at your servant, forenoon and afternoon. As to the latter, I believe he requires very little exercise, as he seldom or ever of late has been seen to move at all...”