Hamilton, Sackville (1732–1818), politician and administrator, was born 14 March 1732 in Ireland, the third son of Henry Hamilton (1692–1743) and his wife, Mary Hamilton (née Dawson). Beginning a career of public service at the age of fourteen, he served as a clerk in the revenue department (1746–62), before being appointed joint principal clerk (1762–7). His administrative abilities were widely acknowledged, and promotions followed: he was successively joint second secretary (1767–72), joint secretary of customs (1772–4), and secretary for port business (1774–80). In 1777 he proposed a scheme for modifying the imperial commercial system but it came to nothing.
On 7 February 1780 he received the most important office of his career when he was appointed under-secretary for the civil department in the Dublin Castle administration. He entered the house of commons that year and was successively MP for St Johnstown, Co. Longford (1780–83), Clogher, Co. Tyrone (1783–95), and Armagh borough (1796–7); he was also returned for Rathcormack, Co. Cork (1783). He rarely spoke in parliament, preferring to observe, analyse, and vote when required. The government expert on finance, he was a key adviser in the Castle, and Thomas Pelham (qv) admitted in 1783 that ‘without Hamilton I think no man in his senses would remain in Ireland an hour’ (Johnston, 59). A firm supporter of Prime Minister William Pitt's commercial propositions, he anticipated the opposition and warned in 1784 about the danger of including an imperial contribution in the proposal. The primacy of Hamilton's position is emphasised by the fact the Pitt sometimes communicated with the viceroy directly through him, and various chief secretaries gave him permission to open all their letters. An amateur architect, he sat on the various building committees in parliament and contributed to designs. As a reward for his work on behalf of Cork trade he was awarded the freedom of the city in 1778; he became a member of the RIA in 1792, and he was said to be a good violinist.
When Lord Fitzwilliam (qv) was appointed lord lieutenant of Ireland in 1795, Hamilton was dismissed as under-secretary, leaving office on the fifteenth anniversary of his appointment. His removal caused much anger as he was regarded, rightly, as one of the key figures underpinning successive administrations, and it contributed to Fitzwilliam's own swift replacement. A former viceroy, Lord Carlisle (qv), informed Fitzwilliam that he had ‘never witnessed the equal of Mr Sackville Hamilton’, but it was precisely because of his ‘very confidential situation’ that he had been dismissed (Johnston, 60). After Fitzwilliam's recall, Hamilton was immediately restored to office on 15 May 1795. He retired from the post 5 June 1796, believing himself vindicated, and became a privy councillor. Out of office he continued to advise the government on financial matters, and during the passing of the Act of Union he was appointed as one of three commissioners to assess the figures for borough compensation.
He married Arabella Berkeley, a daughter of Bishop George Berkeley (qv), and had three sons and two daughters; his second son, Sackville Robert Hamilton, was rector of Mallow, Co. Cork. He died 29 January 1818. Hamilton's long tenure as under-secretary during a critical period in the Anglo–Irish relationship is credited with encouraging continuity in Castle policy under successive administrations. He appears to have been happy to follow instructions rather than pursue his own agenda, and this made him a valuable and trusted advisor, except during the brief and ill-judged viceroyalty of Lord Fitzwilliam.