William's father (Sir Nicholas de Cantilupe (d.1266)) was the 5th and youngest son of William II de Cantilupe (d.1251), 2nd feudal baron of Eaton Bray in Bedfordshire, steward of the household to King Henry III (whose own father William I de Cantilupe (died 1239) had been steward of the household to King John, father of Henry III).