Sir William St Leger (died 1642) was a grandson of Anthony St Leger.
Sir William took part in "the flight of the Earls" in 1607, when Hugh O'Neill, Earl of Tyrone, and Rory O'Donnell, Earl of Tyrconnell, together with more than ninety of their family and followers, the chief of the Gaelic and Catholic resistance in Ireland, fled to Europe. Sir William spent several years abroad. Having received a pardon from King James I and extensive grants of land in Ireland, he was appointed president of Munster by Charles I in 1627. He warmly supported the arbitrary government of Thomas Wentworth, Earl of Strafford, actively assisting in raising and drilling the Irish levies destined for the service of the king against the Parliament. In the great rebellion of 1641 he bore the chief responsibility for dealing with the insurgents in Munster; but the forces and supplies placed at his disposal were utterly inadequate. He executed martial law in his province with the greatest severity, hanging large numbers of rebels, often without much proof of guilt. He was still struggling with the insurrection when he died at Cork on July 2, 1642. Sir William's daughter Margaret married Murrough O'Brien, Ist Earl of Inchiquin; his son John was father of Arthur St Leger, created Viscount Doneraile in 1703.
