Sidney (Sydney), Henry (1641–1704), Viscount Sidney, lord lieutenant of Ireland (1692–3), of London and Kent, was born in Paris c. March 1641, fourth son of Robert Sidney, 2nd earl of Leicester (d. 1677), and his wife Lady Dorothy (d. 1659), daughter of Henry Percy, earl of Northumberland. His elder brothers included Algernon, the famous republican. As part of his education he travelled in Spain and Italy (1658–64) in the care of a Calvinist minister, Dr Thomas Pierce. His mother left him a small estate at her death, while on his father's death in 1677 he received an estate in Warwickshire and £25,000 cash.
In 1665 Sidney was appointed groom of the bedchamber to James (qv), duke of York, and master of horse to the duchess, though by 1666 he had been removed from both positions because of his questionable relationship with the duchess. In 1667 he was appointed captain of foot in the Holland regiment, and in the 1670s progressed through various public and court positions, including envoy to France and master of the robes. In 1678 he was appointed colonel of a regiment of foot. The following year, having been elected MP for Bramber, Sussex, he was sent as envoy to the states general, where he won the confidence of William of Orange (qv). During his time as envoy, he placed himself on the side of those who favoured the exclusion of York from the throne, though he also demonstrated his concern to uphold the rights of succession for William's wife, Mary, should her father be excluded. His activities earned him the displeasure of Charles II, and he was recalled in 1681. William's attempt to counteract the king's displeasure by getting Sidney appointed commander of the English regiments in the Dutch service was unsuccessful, while the accession of James II in 1685 further undermined Sidney's career. However, having remained out of view in Italy in 1686–7, he returned to England in 1688 in time to take a leading role in the conspiracy to bring William to England. He was one of the seven signatories of the letter of invitation, and, having travelled to the Hague in August, returned to England with William in November in command of a regiment. Elected to the convention parliament, in the first months of 1689 he started to reap the rewards of his association with William III: he was appointed privy councillor, gentleman of the bedchamber, and colonel of the regiment of Foot Guards, and in April was raised to the peerage as Baron Milton and Viscount Sidney of Sheppey, Kent.
In the summer of 1690 Sidney accompanied William to Ireland, and was present at the battle of the Boyne. Following William's departure (September), Sidney was appointed as one of the lords justices to govern Ireland. He served in that post till December 1690, when he returned to London to take up office as secretary of state for the northern department. During 1691 he was appointed warden of the Cinque ports, attended William in Flanders, and by the end of the year was being suggested as lord lieutenant of Ireland. In early 1692 he was appointed to that post, though his suitability for the position, and potential for a successful time in office, was called into question almost immediately when accusations were made that he had sold government places when a lord justice. Despite such concerns, he travelled to Ireland in August with orders to summon a parliament. During September he made preparations for meeting that assembly and, in accordance with Poynings’ law, compiled and sent to England a government legislative programme, including two supply bills which he deemed essential given the precarious state of Ireland's public finances. However, he made the mistake of allowing parliament to meet on 5 October before the full body of government legislation had been returned to Ireland, and so quickly found himself facing a major political crisis as the house of commons utilised the time on its hands to investigate the many grievances that had arisen since 1690. These included a general dissatisfaction among protestants with the apparent leniency in the articles of Limerick towards catholics, the suspected embezzlement in the revenue and forfeitures, and the granting of a large part of the forfeited lands to Williamite favourites. In most of these areas Sidney was himself implicated as a previous and present chief governor and as one of those who had benefited most from William III's generosity with the forfeitures. As the investigations of the commons gathered pace, Sidney became greatly concerned at the potential for such activity to result in proceedings against government officials. At the same time, the delay in presenting the government's two supply bills, which returned from London in late October, prompted a constitutional crisis when the commons rejected one of the bills because it did not arise in the lower house, and claimed that the ‘sole right’ to initiate supply legislation lay with the commons, a claim that appeared to challenge Poynings' law and the prerogative of the crown. Within a matter of days Sidney took the opportunity arising from that claim to prorogue parliament (3 November), thereby putting an end to the commons' investigations.
Sidney remained in Ireland for a further eight months, though he failed to dissipate the constitutional crisis, primarily because he pursued a confrontational policy, removing ‘sole right’ advocates from government office and securing the backing of the Irish and English judicial benches for his outright rejection of the commons' claims. At the same time, he realised that he would not be able to solve the crisis. His discomfort grew as Irish affairs, and in particular his own role in such matters, were raised in the English parliament in early 1693, resulting in addresses from that assembly to William III. By the time Sidney was replaced by a commission of lords justices in June 1693, he was glad to leave Ireland and pursue other avenues for his advancement, as master of the ordnance (1693), earl of Romney and lieutenant-general in the army (1694), a lord justice of England in the absence of the king (1697, 1698), and groom of the stole (1700). Like most of the Williamite favourites who received large grants of lands in Ireland, he had sold his holdings of about 50,000 acres by the time the lands were resumed in 1700 by the English parliament.
Removed from his offices on the accession of Queen Anne, he died of small pox, 8 April 1704, and was buried in St James's, Piccadilly. A handsome man who was renowned for being unscrupulous in his sexual relations, he never married, though he was said to have fathered many children.