Sadleir (Sadler), Thomas (d. c.1680), army officer and administrator, was the fourth son of Richard Sadleir of Sopwell, Hertfordshire, and Joyce Honywood of Charing, Kent, and Marks Hall, Essex. He appears as a lieutenant colonel in the parliamentarian army, served under the earl of Manchester in 1643, and was captain of a troop of horse by December 1644. In August 1649 he came to Ireland as adjutant general of foot in the expeditionary force led by Oliver Cromwell (qv), and campaigned under Cromwell in Leinster and Munster in 1649–50. He commanded a detachment that captured four forts in Tipperary and Kilkenny in March 1650, executing his mission with great ruthlessness and professionalism. Until about autumn 1652 he campaigned mainly in west Munster, helping to suppress the remnants of Irish resistance, but in November of that year he became governor of Wexford; one of his first acts in this position was to evict all the Irish from Wexford town. He also acted as sheriff of Wexford in 1652.
In February 1655 Cromwell ordered the transfer of a brigade of 2,600 foot and 600 horse from Ireland to England to contain an anticipated royalist uprising. Sadleir commanded the foot in this force and helped the authorities to arrest royalist plotters around Shrewsbury. After receiving a gold medal and chain for his services, he returned to Ireland in the autumn to become governor of Galway.
During the 1650s Sadleir purchased debentures from his own men to the tune of around £2,300; as his own personal arrears of pay were another £2,300, this meant that he was entitled to lands worth £4,600. In August 1656 he petitioned that these debentures be realised in lands in south Galway and north Clare at his own convenience. However, probably because this would have conflicted with the government's plans to transplant catholic landowners to Connacht, he eventually received lands in Kilnahalagh, Co. Tipperary.
By the mid 1650s he appears to have become a baptist, which caused the authorities to regard him with suspicion. The baptists, who were strongly represented in the army in Ireland, were unhappy with Cromwell's decision to suppress the English parliament and to inaugurate the protectorate. While in England in 1655, Sadleir met and sympathised with the imprisoned republican dissident Edmund Ludlow (qv). As governor of Galway, he allowed the baptists complete freedom to worship and, in 1657, threatened to resign after the local independent minister refused the baptists permission to preach in the church. However, unlike Ludlow and others, he did not openly defy the government and he remained sufficiently in favour to sit as MP for Carlow, Kilkenny, and Wexford in the union parliaments of 1654–5 and 1656–8, and for Galway and Mayo in that of 1659.
Following the collapse of the Cromwellian regime in May 1659 and the return to power of the Rump parliament, the Irish army was ruthlessly purged. His republican reputation still intact, Sadleir survived on 28 June, by forty-eight votes to twenty-two, a challenge in the Rump parliament to his military commission; in October he and other officers in the Irish army accepted the army's seizure of power in England from the Rump. By then the chronic political instability in both England and Ireland had encouraged the conservotive protestant gentry of Ireland to take matters into their own hands. In December Sir Charles Coote (qv) invited Sadleir and his officers to his residence at Tyrellan, slipped away, and then seized Galway city in their absence. This formed part of a larger conservative coup that ousted the radicals from power in Ireland. Sadleir was imprisoned for a time and was formally dismissed from the army after the restoration of the monarchy in 1660. In October of the same year he was arrested in Tipperary on suspicion of plotting against the government. He retired after 1660 to his estates at Kilnahalagh, which he renamed Sopwell Hall, after his birthplace. Thereafter, he rapidly accommodated himself to the new regime, sitting as MP for Tipperary county in the 1661–6 parliament, and serving as commissioner for the poll money ordinance in 1661 and as high sheriff of Tipperary in 1666.
By 1642 he had married Anne, widow of John Shadd and daughter of Thomas Goodridge of St Albans, Hertfordshire. They had a son and three daughters. In May 1662, he married Mary, widow of Vincent Gookin (qv) (d. 1659) and daughter of James Salmon of Glandore, Co. Cork. This marriage effectively signalled his acceptance by his peers as a member of the respectable protestant gentry of Ireland. He died around 1680 and was succeeded by his son Thomas.