William Steuart (also sometimes spelt Stewart) was the second son of Colonel William Stewart (d.1691), adjutant to the Marquess of Montrose at the Battle of Philiphaugh, by his wife Barbara, the granddaughter of James Stewart, Earl of Arran and Lord Chancellor of Scotland.
His father had been granted lands in Ireland by Charles II of England in lieu of arrears of pay due to him as a Cavalier officer during the English Civil War, and it was there that William Steuart grew up.