Sir William St Leger PC (Ire) (1586–1642) was an Anglo-Irish landowner, administrator and soldier, who began his military career in the Eighty Years' War against Habsburg Spain.

His great-grandfather Anthony St Leger (1496–1559), had served as Lord Deputy of Ireland from 1540 to 1548, whereas his father held a number of administrative positions in Munster before he was killed during the Nine Years' War in 1600.

[2] Few details are known of St Leger's career before 1607, when he killed a man in a duel and took refuge from the law with Hugh O'Neill, Earl of Tyrone, one of the leaders of the rebellion in which his father died.

[4] Although Buckingham retained his influence at court following the succession of Charles I, the damage to his reputation was enhanced by an equally disastrous assault on Saint-Martin-de-Ré in 1627 which St Leger also helped organise.

Along with this position, he inherited Buckingham's struggle for control of the province with his local rival Richard Boyle, 1st Earl of Cork, and to counter this, St Leger became an ally of Thomas Wentworth when the latter was named Lord Deputy of Ireland in January 1632.

[7] In 1630, St Leger had proposed a new plantation scheme in County Tipperary, which was never implemented but aligned with Strafford's long-term policy of expanding Protestant cultural and religious dominance in Ireland.

[8] However, in the short term, Strafford was less concerned with religious issues and more focused on increasing royal income by reclaiming lands that rightfully belonged to the Crown and ending widespread corruption among government office holders.

In 1843, Daniel O'Connell quoted him as saying about the harsh policy adopted by the government in Dublin: "The undue promulgation of that severe determination to extirpate the Irish and papacy out of the kingdom, your Lordship rightly apprehends to be too unseasonably published.