The son of Henry Percy, 9th Earl of Northumberland, Algernon became a peer in his father's lifetime, as Baron Percy in 1626. During the years immediately preceding the Civil War he served as an admiral, making earnest but unsuccessful efforts to reform the navy, and in 1637 he was made lord high admiral of England. In 1639 King Charles I appointed him general of the forces north of the Trent, and a member of the council of regency.
Northumberland played a distinguished and honourable part in the troubled times of the Civil War. He was a friend of Strafford, and gave evidence at his trial which, though favourable on the important point of bringing the Irish army to England, was on the whole damaging; and he afterwards tended more towards the popular party, of which he soon became leader in the House of Lords. He was a member of the committee of safety, and later of the committee of both kingdoms; and he took an active part in the attempts to come to terms with the king, whom he visited at Oxford for that purpose in 1643 and at Uxbridge two years later.
Northumberland helped to organize the New Model Army; and in 1646 he was entrusted by parliament with the charge of the king's younger children. He led the opposition in the House of Lords to the proposal to bring Charles I to trial, and during the Commonwealth he took no part in public affairs. At the Restoration he became a member of Charles II's privy council, and with his habitual moderation he deprecated harsh proceedings against the regicides. His second wife, Elizabeth (d. 1705), daughter of Theophilus Howard, 2nd Earl of Suffolk, brought him Northumberland House in the Strand, London, which was demolished in 1874 to make room for Northumberland Avenue.
| Preceded by: Henry Percy | Earl of Northumberland | Followed by: Joceline Percy |