Sidney, or SYDNEY, ALGERNON, grand-nephew of the famous Sir Philip, was born probably at Penshurst, Kent, and in 1622, the second son of Robert, second Earl of Leicester (1595-1677). He received a careful education, and accompanied his father in 1632 on his embassy to Denmark, and in 1636 to France. In 1641-43 he commanded a troop of horse against the rebels in Ireland, of which country his father was (nominally) Lord-lieutenant. Then with his elder brother, Viscount Lisle, he returned to England, and, declaring for the parliament, was in March 1644 appointed to a troop in the Earl of Manchester's regiment. At Marston Moor he was severely wounded; in 1645 was appointed governor of Chichester, and returned by Cardiff to parliament; in 1646 attended his brother, now Lord-lieutenant, to Ireland as lieutenant-general of the horse and governor of Dublin; and in 1647, after receiving the thanks of the House of Commons for his services, was appointed governor of Dover. In 1649, though nominated one of the commissioners, he kept himself clear from any hand in the king's trial, which yet he justified on abstract grounds, speaking afterwards of the execution as 'the justest and bravest action that ever was done in England or anywhere else.' In principle a severe republican, he resented Cromwell's usurpation of power, and from the dissolution to the restoration of the Long Parliament (1653-59) lived in retirement at Penshurst. He then was nominated one of the Council of State, and next was engaged for a twelvemonth on a political mission to Denmark and Sweden. After the Restoration he lived precariously on the Continent, fitting from place to place (Rome, Brussels, Augsburg, Montpellier, Paris, &c.); but in 1677 a pardon was procured for him from Charles II., and he returned to his native country. In 1679 he twice stood for parliament, but each time was jockeyed out of his seat in favour of the court candidate; and an attempt was made that same year to involve him in the sham Meal-tub Plot. The attempt miscarried; still, he deemed it prudent to retire for a while to France, where he bought a small property, and, to detach Louis XIV. from Charles, entered into negotiations with him through Barillon. That prior to this he had taken moneys from the French ambassador, either for himself or (more likely) for the republican cause, is admitted by Hallam and Macaulay, but disputed by Mr Ewald, who contends that Barillon embezzled the thousand guineas that he set down to Sidney's account. Anyhow, to understand Sidney's relations with Louis, it must be borne in mind that he was hardly less hostile to William of Orange, as stadtholder, than to Charles himself, as king. Next year he was back in England, and, it is said on somewhat doubtful authority, assisted his friend, William Penn, to draw up the Pennsylvanian constitution, including the ballot, universal suffrage, the abolition of a property qualification, religious equality, prison reform, and the abolition of capital punishment except for murder and treason. In June 1683, when the Rye-house plot was announced, the chance was seized to get rid of men felt to be dangerous, and, along with Lords Russell, Essex, and Howard, Sidney was arrested and committed to the Tower. On 21st November he was tried for high-treason before the brutal Jeffreys, and, on no evidence but the traitor Lord Howard's and his own unpublished Discourses concerning Government, was found guilty and sentenced to die. He met his doom bravely on Tower Hill, 7th December, and was buried the next day at Penshurst. His attainder was reversed in 1689; his Discourses appeared first in 1698.
See Blencowe's Sidney Papers (1813), and the Lives of Sidney by S. W. Meadley (1813) and A. C. Ewald (2 vols. 1873), with other works cited at RUSSELL, SHAFESBURY, and CHARLES II.