Knox, Thomas (c.1641–1728), merchant, landowner, and privy councillor, was eldest among three sons and two daughters of Thomas Knox (d. 1685), merchant and burgess of Glasgow, and his wife Elizabeth, daughter of Andrew Spang, a burgess of Glasgow of Danish origin. He settled in Belfast by the mid 1660s and became a merchant stapler of the town in 1670. He was active as a merchant, with shares in several ships, and became a burgess of Belfast in 1680. He also became a burgess of Glasgow in 1685, on the death of his father. Knox married (probably in the late 1670s) Mary (c.1651–1717), daughter of Robert Brice (d. 1676), of Castle Chichester, Co. Antrim, merchant and JP, and his wife Elizabeth, who was probably a Stewart of Ballintoy. His wife's brothers were Randal Brice (c.1646–97), MP for Lisburn, and Col. Edward Brice (c.1659–1742), MP for Dungannon.
Knox became one of the most prosperous merchants in Belfast. His success and status led to his being appointed high sheriff of Co. Antrim in 1685, although he was not the first Belfast merchant to hold this office. The appointees for that year, the first of the reign of James II (qv), included some dissenters, but on Knox's being described as ‘a presbyterian Scotch whig’, Lord Clarendon (qv) replied ‘The character must be given out of prejudice and particular pique, for there is not the least shadow of truth in it; this person being notoriously known to be a constant frequenter of the church, and never resorted to any conventicle since he lived at Belfast, where he is the most considerable merchant’ (S. W. Singer (ed.), The correspondence of Henry Hyde, earl of Clarendon, i (1928), 287).
Although Knox himself was a member of the Church of Ireland, like most Scots at that date he had relatives who were presbyterians, the most notable of whom, in Ulster, was his brother-in-law, Edward Brice, who married Dorothea, daughter of Arthur Upton (qv) of Castle Upton, Co. Antrim, and through this marriage was connected to many leading presbyterian families in Ulster. It was presumably because of his presbyterian connections that Thomas Knox was named as one of the burgesses of Belfast in the new charter of 16 October 1688, one of a series of charters designed to give James II control of urban parliamentary elections. In December 1688 Knox was one of the gentlemen who sent copies to Dublin of a mysterious letter to Lord Mount Alexander, dropped in the street in Comber, Co. Down, warning of an impending massacre of the protestants by the Roman catholics. Knox, like other Belfast burgesses, subscribed to the Northern Association against James II, and probably left Ireland with his wife and family after the Association was defeated at the Break of Dromore (March 1689). He spent at least a year in Glasgow, where his daughter Penelope (1689–96) was born, and where his brother William Knox (c.1652–1728) was a burgess.
He returned to Belfast in 1690, was MP for Newtown in Co. Down (1692–3), and commissioner of array for Co. Antrim in 1693. In the previous year he had purchased a substantial estate at Dungannon in Co. Tyrone from Arthur Chichester, 3rd earl of Donegall (1666-1706). He appears to have settled permanently at Dungannon by 1697, when he resigned his Belfast burgessship in favour of his brother-in-law, Edward Brice. He ceased to operate as a merchant although he continued to invest in trading ventures.
Knox was MP for Dungannon 1695–1727, and high sheriff for Co. Tyrone in 1702. Politically he was a whig, and he was appointed a privy councillor in 1715, after the accession of George I. He died 11 May 1728, and is buried in the church of St Ann, Dungannon. Of his two daughters who survived to adulthood, Mary married Oliver St George, and Jane married Charles Echlin. His estates devolved on his nephew Thomas, son of his brother John Knox (d. 1718) of Ballycreely and Ringdufferin, Co. Down, who was the ancestor of the earls of Ranfurly.