Bentinck, William Henry Cavendish (1738–1809), 3rd duke of Portland , viceroy of Ireland, was born 14 April 1738 at Bulstrode, Buckinghamshire, eldest son of William, 2nd duke of Portland, and his wife Lady Margaret Cavendish Harley. Educated at Westminster and Christ Church, Oxford, he graduated MA (1757). He entered the commons in 1761, and became duke of Portland in 1762. An ally of the marquis of Rockingham, he sat in his first cabinet (1765–6), and afterwards was a leading opposition whig till the fall of North's ministry (1782).
Influenced by his cousins the Ponsonbys, Ireland's leading Patriot family, he was interested in Irish affairs, and Rockingham appointed him lord lieutenant of Ireland on 14 April 1782. He believed strongly in the interdependence of Ireland and Great Britain, and wished to remedy Irish grievances only to bind the two kingdoms together more closely. Instructed to stall agitation for legislative independence by adjourning parliament, he found this impossible given the strength of popular feeling, noting that he now had to manage not just the Irish parliament, but the entire country. However, his policy – basing his administration on an Irish whig party under Ponsonby leadership by replacing existing officeholders with Patriots – further stoked expectations. He welcomed William Ogilvie’s (qv) plan for a carefully defined federal union which would acknowledge London's supremacy in matters of state and general commerce, but found that such a moderate settlement was unacceptable to Patriot opinion, as was his own suggestion for a commission to fix the limits of legislative independence. After years of exploiting Irish grievances in opposition, the Rockingham whigs were finally being brought to account by their Irish allies, and on 27 May 1782 Portland was compelled to announce that Westminster would legislate for Irish parliamentary independence. He warned London that any delay could endanger the British connection: in June legislation was passed at Westminster to repeal the 1720 declaratory act, and in July in the Irish parliament to amend Poynings' law and the perpetual Irish mutiny act, and guarantee the Irish judiciary's independence – the main planks of the ‘constitution of 1782’. With the whig ministry's resignation after Rockingham's death, Portland was replaced as viceroy by Temple (qv) on 15 August 1782.
In April 1783 he was appointed compromise prime minister of the Fox–North coalition, and through the new Irish viceroy, Robert Henley, 2nd earl of Northington (qv), he again attempted to create a Ponsonby whig party, but soon abandoned his efforts having concluded that Patriots made poor officeholders. Fearing any advance beyond the settlement of 1782, he viewed the Volunteer reform conventions of 1783 as a serious threat to the British connection, and advised Northington to arrest the ringleaders. Resigning as prime minister in December 1783, he led the Rockingham whigs in opposition. He hated public speaking and relied on Fox and Edmund Burke (qv) as his spokesmen and parliamentary managers. In 1785, however, he proved an effective opponent of Pitt's commercial propositions, encouraging English manufacturers to deluge Westminster with petitions.
Leader of the conservative whigs who opposed parliamentary reform and supported war with France, in 1794 he accepted Pitt's offer of coalition and became home secretary, which included supervising Irish affairs and allowed him to work for his long-held ambition that Ireland might be made ‘a peaceful and useful member of the British empire’ (Bolton, 15). He chose his close friend and political ally Earl Fitzwilliam (qv) as viceroy but Fitzwilliam exceeded his instructions by encouraging catholic hopes for emancipation and dismissing several important officials, much to Portland's embarrassment. A staunch protestant, Portland believed that immediate emancipation, and Fitzwilliam's plans to create a yeomanry force open to catholics, risked undermining protestant ascendancy in Ireland. Under pressure from the cabinet, he advised Fitzwilliam to resign (21 February 1795). Fitzwilliam's recall was seen as a betrayal by Irish reformers and catholics, and Portland received much of the blame. He instructed the new lord lieutenant, Camden (qv), to make no further concessions to catholics and to rely on the protestant interest. From 1795 he viewed the growth of the United Irishmen with increasing anxiety and gave Dublin Castle free rein to suppress them.
From 1798 to 1800 his relations with the viceroy, Cornwallis (qv), were often strained; there were tensions between the two going back to the American war. He believed that Cornwallis was too lenient to the defeated rebels and too sympathetic to catholics. Portland gave strict instructions that catholics should receive no definite offer of emancipation but merely the hope that a united parliament would look sympathetically at their claims. Although he supported Cornwallis and Castlereagh (qv) in carrying the union, he believed that the price paid in peerages and honours was excessive.
His home secretaryship (1794–1801) was probably his most successful office. Although a conscientious administrator, Portland could be aloof, punctilious, and procrastinating, and even his admirers made no great claims for his ability: Burke thought him a ‘virtuous, calm, steady character’ (Charlemont MSS, ii, 106), while his biographer admitted he was not ‘a great or even very able man, but . . . was essentially honourable and high-minded’ (Turberville, ii, 321); in contemporary caricatures he often appears as a block of Portland stone.
Cool towards Pitt's proposals for catholic emancipation, after Pitt's resignation in February 1801 Portland joined Addington's government as lord president of the council, and held the same post when Pitt returned in 1804. He retired after Pitt's death, but when the ‘Talents’ ministry fell in 1807 he reluctantly formed a government, taking office on a strong ‘no popery’ platform. He was prime minister in name only, and government devolved on Canning and Castlereagh. After their duel (21 September 1809) he insisted on resigning. He died 30 October 1809 at Bulstrode.
He married (1766) Dorothy (1750–94), the only daughter of William Cavendish, 4th duke of Devonshire (qv); they had four sons and two daughters.