Thomas was the son of John Stewart, earl of Angus and Margaret Abernethy. He was very young when his father died in 1331 and only emerged as an active figure c.1350. Thomas married Margaret, daughter of William Sinclair of Roslin c.1353. In the following year he was described as lieutenant in Angus and the Mearns, presumably acting for his kinsman, Robert the Steward, who was guardian of the realm at that time. Thomas led an amphibious assault on Berwick in 1355 and served as royal Chamberlain 1356-8. Was imprisoned in Dumbarton Castle in 1361 because of his suspected involvement in the assassination of David II’s mistress Katherine Mortimer. Was still in custody the following year when he died, allegedly as a result of contracting the plague.
Angus and his wife, Margaret, were survived by two daughters, Margaret and Elizabeth. Margaret the elder married Thomas, earl of Mar, and after his death in 1377 she became the mistress of William Douglas, first earl of Douglas, and produced a son, George Douglas, by him. Despite George’s illegitimacy he was eventually recognised as heir to his maternal grandfather’s earldom of Angus.