Sinclair, Thomas (1838–1914), merchant and politician, was born 23 September 1838 in Belfast, son of Thomas Sinclair (1811–67), merchant and shipowner, and his wife Sarah (1800–49), daughter of William Archer of Hillsborough, Co. Down. Thomas Sinclair the elder was born and educated in Belfast. With his brother John Sinclair (1808–56), he set up a provisions and general merchant store at 5–11, Tomb St., Belfast. John married Eliza Pirrie of a prominent shipbuilding family, and this may have contributed to both brothers becoming shipowners. Thomas was president of the Belfast chamber of commerce and was appointed chairman of the Belfast harbour commissioners in May 1863. He died in office at the house of his brother-in-law, Samuel Gibson, in London on 2 January 1867; his remains were shipped to Belfast, where he was buried in the New Burial Ground. He was predeceased by his brother, his wife, and a son and daughter. Only Thomas the younger survived him, inheriting some £35,000.
The younger Thomas was educated at the RBAI and QCB, graduating BA (1856) with a gold medal for mathematics, and MA (1859) with gold medals in logic, political economy, and English literature. On his father's death he took over the family general provisions business and further increased its prosperity by amalgamating with Kingan & Co, a large provisions concern with extensive American branches, owned by Samuel Kingan of north Co. Down, who was married to Thomas's cousin, Sarah, daughter of John Sinclair. Kingan's brother, John, established the Kingan Mission to the Deaf and Dumb in Belfast, of which Thomas Sinclair was a supporter.
Sinclair refused requests to stand for parliament, but as a wealthy and prominent Belfast citizen who was DL, JP, president of the Ulster chamber of commerce (1876, 1902), and privy councillor (1896), he played a highly influential role in the affairs of the province. Originally a supporter of W. E. Gladstone, he joined the Ulster Liberal party in 1868 and supported the 1870 and 1881 land acts. The 1886 home rule bill provoked him to his first act of defiance: he convened a large meeting of liberals in the Ulster Hall on 30 April of that year and passed a resolution condemning the bill. He then formed the Ulster Liberal Unionist Association and, in his capacity as its chairman, organised the Ulster Convention, held in Belfast on 17 June 1892, which brought together 11,879 Ulster unionists of every creed, class, and party in the first mass protest against home rule. Nine months later he was appointed to the executive committee of the new unionist clubs, hastily founded throughout the province to protect the union. However, the Liberal defeat of 1895 lessened the threat of home rule, the unionist clubs were temporarily suspended, and Sinclair returned to his liberal reformist activities. From 1895 he was a leading Ulster member of the recess committee originated by Horace Plunkett (qv), and was a steady support to Plunkett, backing him in his choice in 1899 of the nationalist T. P. Gill (qv) as secretary of the Irish Agricultural Organisation Society, despite the strong protest of conservative unionists.
However, the 1904 devolution crisis saw Sinclair joining forces with those same unionists. He was one of thirty members of the standing committee of the Ulster Unionist Council on its formation in December 1904, and devoted the rest of his life to defending the union. In January 1911 he proposed that the suspended unionist clubs should reorganise, extend their membership, and take on new responsibilities. This translated to the clubs taking up arms: by April over eighty were drilling and had formed the basis for the UVF. A succinct and cogent writer, Sinclair was appointed on 25 September 1911 to a commission of five to frame a constitution for a provisional government of Ulster. The following year his was among the clearest and best argued of the contributions to the unionist party's collection of essays, Against home rule (1912). Identifying Ulster as the six counties, he advanced the two-nation theory in refutation of nationalist claims and warned that Ulster would certainly resist until the end. Later that year he was given the crucial task of drafting the text of the Solemn League and Covenant, which pledged signatories to defeat home rule and refuse to recognise the authority of an Irish parliament. This was signed by 471,414 people in the Belfast city hall on 28 September 1912. Sinclair did not live to see the outcome of his struggle; he died at his home, Hopefield House, Belfast, on 14 February 1914 after a prolonged illness. His funeral to the city cemetery on 18 February was headed by a procession of 200 UVF officers. Sinclair's portrait, by Frank McKelvey (qv), is in the Ulster Museum.
Concerned with religious and educational matters, he framed the financing proposals on what became known as the commutation and sustentation funds after the withdrawal of the regium donum from the presbyterian church in 1869. As chairman of the convocation of the Queen's University, he resolutely defended its non-sectarian principle and rejected denominational teaching in any form. Through his position on the recess committee, he ensured that the system of vocational and technical education administered by the new Department of Agriculture would be secular.
Sinclair married first (1876) Mary Duffin (d. 1879) of Strandtown Lodge, Belfast, who came from another family prominent in the mercantile, cultural, and public life of Belfast, and secondly (1882) Elizabeth, daughter of William Richardson of Brooklands, Belfast, niece of John Grubb Richardson (qv), and widow of Thomas's cousin John M. Sinclair. He had four sons and three daughters, the eldest of whom, Frances Elizabeth Crichton (1877–1918), was educated privately, travelled in Europe, and after her marriage to W. S. Crichton (benefactor of Liverpool University) lived in Liverpool and took up writing as a recreation. The precepts of Andy Saul, The soundless tide (1911), and Tinker's Hollow (1912) are pleasant sketches of Ulster character and speech; she also wrote for children. She died in England on 23 November 1918.