Sinclair (Sinclaire), William (1760–1807), linen manufacturer and radical, was second among four sons of Thomas Sinclair (1719–98), merchant, and his wife Hester Eccles Pottinger, whom he married in 1749. Thomas came to Belfast from Newtownards and served his apprenticeship with Robert Armstrong, a grocer in High St., before inheriting an extensive linen business, including a large home and adjoining premises in Belfast town centre and three bleaching greens on the western fringes of the town (Mountain Green, Lodge Green, and Falls Green), on the death of his elder brother John in November 1769. He moved from his house in High St. to Mill St., where his home gained the reputation of being one of the most hospitable in Belfast. As well as William, he fathered three other sons; Thomas, George, and John Sinclair (1763–1857). The first son, Thomas, may have died young, for William, as second born, took over his father's bleach mills. According to William Drennan (qv) ‘his head appeared of the shrewd, intelligent kind peculiar to the Belfast merchants, gumption without cultivation’ (to Martha McTier, 27 February 1807; Chart, Drennan letters, 374). In October 1783 he was a founder member of the Belfast chamber of commerce, of which he was later (1804) the president of its council. In 1784 he pioneered the use of American potash in the bleaching of linen and by the early 1790s his extensive bleachworks at Ligoniel were reputed to be the largest, most mechanised, and most technically advanced in the region. Theobald Wolfe Tone (qv), who referred to Sinclair in his journals as ‘the draper’, reported how Sinclair had to overcome the resistance to innovation shown by many Belfast townsfolk. In 1801 Sinclair invested £2,500 in another linen business. He combined these enterprises with acting as land agent for David Ker's substantial estates in Co. Down.
Aged only 25 he built an impressive house for himself and his family in Donegall Place, the most fashionable and expensive thoroughfare in Belfast, and as a leading figure in the town was involved in setting up the Belfast Society for Promoting Knowledge, which latterly became the Linen Hall Library (he and his brother John were intimates of Thomas Russell (qv), librarian of the society 1794–6). He was active in the first and third presbyterian congregations in Rosemary Lane, signed the prospectus for the foundation of the Belfast dispensary (13 April 1792), and on 16 May 1792 was appointed to its committee. In 1802 he further donated £50 towards a school for young girls. He was interested in field sports and collected a menagerie of hawks, hounds, horses, and fighting cocks in the ground to the rear of his home. This must have been a family trait; his brother John also kept a pack of hunting hounds and a monkey at his home, which was situated opposite his brother's, at the west side of Donegall Place.
Both William and John were politically engaged. William helped organise the Volunteer convention at Dungannon in 1782 and was active in the Volunteer campaign for parliamentary reform 1784–5. He was a leader of the First (Green) company of the Belfast Volunteers, which had admitted catholics from August 1791. He corresponded with Tone and Russell in the summer of 1791, and on 4 October 1791 chaired a meeting of the three Belfast Volunteer corps who pledged their joint efforts to obtain parliamentary reform and resolved to set up a committee to open correspondence with catholics. He was a member of the secret committee whose meeting with Tone in Drew's tavern, Belfast, on 14 October 1791, led to the establishment of the first society of United Irishmen, of which Sinclair was chairman (18 October). His brother John also joined the United Irishmen. When the Dublin Society of United Irishmen was formed (9 November 1791) William was voted in as an honorary member and continued to correspond with the Dublin members.
At the Bastille celebrations in Belfast on 14 July 1792, William Sinclair and Tone argued for a declaration in favour of catholic emancipation when others, including Theobald McKenna (qv) and James Napper Tandy (qv), feared radical proposals might be rejected. That afternoon Sinclair moved the adoption of the address to the National Assembly of France, which Tone had prepared. It was carried unanimously. He then rose again and read the call for full catholic emancipation. A hot debate commenced, which did not terminate until six o'clock. A vote was taken, with only five in opposition; an outstanding victory for the United Irish cause. As secretary of the Northern Whig Club, of which he was a founding member in 1790, Sinclair proposed resolutions drawn up by Tone (5 November 1792), which were published, to the effect that the club was in favour of catholic relief.
On 16 January 1793 he signed a resolution to call a meeting in Belfast to thank George III for putting catholic relief before the Irish parliament. At the Dungannon convention (15 February 1793) he was one of the representatives for Antrim and advanced the view that a date for appointing a national convention on reform should be delayed, as catholics could not be expected to declare their views until catholic emancipation was safely through parliament. He also argued unsuccessfully for a declaration against the war with France.
His views became more extreme as the 1790s progressed: c.1795 he became a member of the Belfast Society of Irish Jacobins, and in 1796 an informer stated that Sinclair was a member of the executive directory which controlled all the United Irish societies in Ulster. On 2 February 1797 Martha McTier (qv) reported that Sinclair had been assured that the French would land in Ireland, and in March 1797 he subscribed at least five guineas for the relief and support of political prisoners. He does not appear to have been involved in the fighting in the summer of 1798, although he attended Wolfe Tone's trial in Dublin (10 November 1798). After 1798 he distanced himself from the United Irishmen. In 1799 he entertained Lord Cornwallis (qv) in the course of a visit to Belfast and then escorted him on a tour of his bleachyards. In 1803 he was one of the three lieutenants of the Belfast Volunteer infantry. He died 11 February 1807.
William married (August 1784) Charlotte Pollock (1763–1850), sister of John Pollock (qv), crown solicitor. She was nominated on 23 December 1793 to the committee of the new lying-in hospital for Belfast, and was treasurer in 1798. She died 9 January 1850, aged 86. Of their three daughters, Elizabeth (1785–1857) married (13 June 1809) the Rev. Edward May, rector of Belfast, brother of Sir Stephen May, and brother-in-law to the 2nd marquess of Donegall (qv); Charlotte married (26 August 1826) Conway Richard Dobbs of Castle Dobbs; and the third daughter married the Rev. James Strange Butson, archdeacon of Clonfert. His brother John died 29 June 1857, the last surviving Volunteer at the time of his death. He had married Margaret (c.1769–9 January 1839), only child of John Clarke; they had four sons and two daughters. The Sinclair family was one of the nexus of interrelated whig families who dominated the liberal party in Belfast from 1832 to 1886.