Clotworthy, Sir John (d. 1665), 1st Viscount Massereene and politician, was the son of Sir Hugh Clotworthy who, together with a number of other Devon gentry, had come to Ireland with the army in the 1590s. Hugh Clotworthy married Mary, the daughter of Roger Langford, also an ex-soldier who had served in the Irish army. On his father's death in 1630 Sir John Clotworthy inherited the family estate around Antrim town. He had been knighted in 1626 and represented Co. Antrim in the Irish parliament of 1634–5. He also held interests in the Drapers’ lands as part of the Londonderry settlement in the Ulster plantation, and he was the brother-in-law of Edward Rowley, the agent for the Londoners. Before 1643 he married Margaret, the daughter of Roger Jones (qv), 1st Viscount Ranelagh.
In the 1630s Clotworthy came into conflict with the lord deputy, Thomas Wentworth (qv). Wentworth tried to strip him of his monopoly of licensing taverns in Co. Antrim and refused to grant him a commission in the company of horse, previously commanded by Sir Hugh Clotworthy. He lost his lease to the Drapers’ lands in 1635 when the plantation land was resumed by the crown because of the Londoners’ failure to carry out their promised settlement. A subsequent attempt to gain control over the Londoners’ lands was blocked by Wentworth. The second area of conflict was over religion. Clotworthy and his wife were convinced presbyterians, and he maintained a presbyterian minister, John Ridge (qv), at Antrim. Wentworth's clampdown on presbyterians had such an effect on Clotworthy that in 1635 he discussed with John Winthrop's son the possibility of emigrating to New England.
In late 1638, as the political situation in Scotland became more unstable as a result of the imposition of the prayer book, Clotworthy appears to have been in Edinburgh, and he may have acted as the link between the Scots and English puritans unhappy with the actions of Charles I. He was certainly doing so by the following year when some of the letters were intercepted. His sister was married to the English puritan and parliamentarian John Pym, a connection that gave him access to the godly group in England. In August 1639 Clotworthy was back in Dublin. In the following year he became MP for the Cornish borough of Bossiney in the Short Parliament, and in November 1640 he was returned as MP for the Essex borough of Maldon in the Long Parliament. His position as an Irish landowner in the Westminster parliament meant that he was uniquely placed to act as the intermediary between Ireland and England in the trial of Wentworth, now earl of Strafford. It was Clotworthy who seconded Pym's motion in November 1640 that a committee of the whole house should be appointed to consider Irish affairs. He became an important figure in orchestrating Strafford's trial and testified against him on a number of occasions, including his treatment over the Londonderry lands and Strafford's attempt to develop Irish linen manufactory.
Clotworthy was back in Ireland by 1642, commanding a regiment in the war that had broken out in late 1641. His main area of military activity was around Lough Neagh, relieving the English held prisoner at Moneymore in Co. Londonderry and recapturing Mountjoy, Co. Tyrone. He also advanced £1,000 under the Adventurers’ Act of 1642 as a speculation to acquire land at the end of the war. By the middle of 1643 he was again in London and proposed the inclusion of Ireland in the Solemn League and Covenant, then being negotiated with the Scots. However, he was not satisfied with the conduct of the Irish war, which he felt was not moving fast enough. In 1645 he was on the parliamentary committee for the relief of Ireland and on the committee for both kingdoms. In October 1646 he was in Dublin negotiating with James Butler, marquess of Ormond (qv), for the surrender of the city to parliament. With the rise of the Independent faction in the English parliament, Clotworthy and his presbyterian faction became increasingly isolated. He was charged by parliament with embezzlement, with obstructing Viscount Lisle (qv) (the parliamentary commander in Ireland), and with treacherous dealings with Ormond. He and eleven others tried unsuccessfully to defend themselves, and Clotworthy fled to Netherlands. His ship was intercepted and he was returned. In January 1648 he was disqualified from sitting in the commons, but by then he had escaped to Holland. He returned to England during the second civil war, which saw the restoration of the presbyterians to influence. After the war he was purged again and was imprisoned, charged with complicity in the Scottish invasion during the war. He remained a prisoner until November 1651. In 1653 he was living at St Martin in the Fields in London.
During the 1650s Clotworthy amassed a considerable landed estate, both as the result of his original investment under the Adventurers’ Act and by subsequent speculation. From the middle of the 1650s there are signs that he became a more respectable figure as the influence of Old Protestants grew under Henry Cromwell (qv). In 1654 he was one of the committee established to determine differences among the Adventurers for Irish land and in the same year he interceded with the lord deputy, Charles Fleetwood (qv), on behalf of presbyterian ministers in Antrim and Down. He acquired a number of local offices. In 1656 he proposed the establishment of a college at Antrim, perhaps intended, as it was later proposed, to train presbyterian ministers.
Clotworthy became a pivotal figure in the process of restoring the monarchy in 1660. He was elected to represent Co. Antrim in the Convention of 1660 and in March was sent to England as one of its agents to represent the interests of the Adventurers and soldiers in Ireland. It was he who unsuccessfully proposed an act confirming such estates as they stood on 7 May 1659. He returned to Ireland and was actively involved in the Convention's debates on the need for an Act of Oblivion. On 21 November 1660 he was created Viscount Massereene and in the following month was appointed to the privy council. He remained a staunch presbyterian and protected individual presbyterian ministers and the presbyterian congregation at Antrim town from government harassment. In his latter years he developed an interest in scientific inquiry. In the later 1650s he was on the fringes of the Hartlib circle and in 1663 he became a fellow of the Royal Society. He died 23 September 1665 in Dublin, survived by his wife. His only child, Mary, married Sir John Skeffington (qv), who succeeded to the Massereene title in 1665.