The highest-ranking officer to support the Levellers, Rainsborough fought for Parliament on both land and sea. He was the eldest son of Captain William Rainsborough, a Levant merchant and naval officer, and inherited his father's property in Southwark, London. Rainsborough invested in the Irish Adventurers' scheme in 1642, in the expectation of gaining land in Ireland when the Irish Uprising had been suppressed. On the outbreak of the First Civil War, he joined the Parliamentary fleet under the Earl of Warwick. In 1643, he took command of the 34-gun frigate Swallow and captured a Royalist vessel carrying supplies and reinforcements to the King. As captain of the Lion later that year, he was active in supporting the Fairfaxes in the defence of Hull. On 11 October 1643, Rainsborough led a force of sailors and musketeers in a raid that captured several Royalist siege guns, forcing the Royalists to abandon the siege the next day.
Transferring to the army, Rainsborough commanded a regiment in the Eastern Association under the Earl of Manchester. His regiment included several officers and a high proportion of men from the American colonies who had returned to England to fight for Parliament. His family was connected by marriage to the WInthrops of Massachusetts — his sister Martha was briefly married to John WInthrop, governor of Massachusetts, while another sister, Judith, was married to Winthrop's son Stephen, who fought in Rainsborough's regiment.
In May 1645, Rainsborough became a colonel in the New Model Army. He captured Gaunt House near Oxford on 1 June 1645 and fought at Naseby under Major-General Skippon. During the New Model's march into the west, Rainsborough distinguished himself at the battle of Langport when he led 1500 musketeers in an attack on the Royalist position. He fought at the sieges of Bridgwater, Sherborne and then Bristol, where Rainsborough's regiment led the storming of Prior's Hill Fort. Rainsborough captured Berkeley Castle, and then besieged Corfe Castle for a time, before being sent to blockade Oxford in December 1645. After the surrender of Oxford in June 1646, Rainsborough took over the siege of Worcester, which surrendered to him on 22 July 1646. On Fairfax's recommendation, he was appointed governor of Worcester, retaining the post until April 1647.
Rainsborough was elected recruiter MP for Droitwich in Worcestershire in January 1647, but still continued with his military duties. Parliament intended to deploy his regiment for the recovery of Jersey. In Rainsborough's absence, however, his troops mutinied at Portsmouth in May 1647 in protest at Parliament's plans for the disbandment of the New Model Army without settlement of the soldiers' grievances. The mutineers marched for Oxford, intending to seize the Army's train of artillery, until Rainsborough joined them at Abingdon and succeeded in pacifying them.
Now deeply involved in the Army's political activities, Rainsborough was one of the delegation of officers that presented the Heads of the Proposals to King Charles in July 1647 as a basis for a negotiated settlement. Rainsborough turned implacably against the King when he contemptuously rejected the Army's proposals. In August 1647, Presbyterian MPs tried to foment a counter-revolution by raising the City of London against the New Model Army. Rainsborough led the advance guard when the Army marched to occupy London.
During October and November 1647, Rainsborough was a leading speaker at the Putney Debates, where he sided with the Leveller radicals, calling for the Army and Parliament to break off negotiations with the King and to force through a new constitution on their own terms. The Grandees Cromwell and Ireton were opposed to this, but within three months the King's intransigence had forced Parliament to adopt Rainsborough's proposal in the Vote of No Addresses. Rainsborough also argued for manhood suffrage ("one man, one vote"), again clashing with Cromwell and Ireton who regarded the idea as tantamount to anarchy. At the Corkbush Field rendezvous in November 1647, Rainsborough attempted to present a copy of the Levellers' manifesto: An Agreement of the People to General Fairfax, but he was waved aside.
In January 1648, Rainsborough returned to naval service. He was appointed vice-admiral in place of the Presbyterian William Batten and given command of a squadron guarding the Solent and Isle of Wight, where King Charles was held prisoner. However, Rainsborough's radical views were unpopular in the Navy where many officers were Presbyterian in sympathy. On the outbreak of the Second Civil War in the spring of 1648, a number of Parliament's warships declared for the King in a naval revolt at the Downs anchorage. Rainsborough was seized by the crew of his flagship, the Constant Reformation, and put ashore. Parliament re-appointed the Earl of Warwick in his place to restore the loyalty of the seamen. With his authority in the navy at an end, Rainsborough transferred back to the army and took command of the Tower of London Regiment at the siege of Colchester. He was one of the commissioners who counter-signed the articles of surrender at Colchester and, at the head of his new regiment, was one of the first to enter the town.
After the fall of Colchester, Fairfax ordered Rainsborough to march north to the siege of Pontefract Castle, intending to place him in command of Parliament's forces in Yorkshire, thus keeping him well away from the centre of political power in London. The Parliamentarian commander in the region, Sir Henry Cholmley, bitterly objected to Rainsborough's appointment and refused to accept his authority. Rainsborough and the Tower Regiment quartered at Doncaster while the wrangling continued. On the night of 30 October 1648, a party of four Royalists from Pontefract gained admission to Rainsborough's lodgings and attempted to take him prisoner. Rainsborough refused to accompany them and in the ensuing struggle, he was run through with a sword and killed. Many believed that Sir Henry Cholmley was implicated in Rainsborough's death because Cholmley's troops had failed to prevent the cavaliers from leaving Pontefract or from entering Doncaster and finding Rainsborough's lodgings. Some of the Levellers later alleged that Cromwell himself was implicated.
As the senior Leveller supporter in the Army, Rainsborough's death was a severe setback for the movement. His funeral in London occasioned a massive Leveller-led political demonstration, with thousands of mourners wearing ribbons of sea-green in his memory, which was thereafter adopted as the Levellers' colour.
References:
Bernard Capp, Cromwell's Navy, the Fleet and the English Revolution, 1989
S.R. Gardiner, History of the Great Civil War, vol. iii, vol. iv
Ian J. Gentles, Thomas Rainborowe, Oxford DNB, 2004