He was the fourth son of Isabella of Angoulême, widow of John, King of England, and her second husband, Hugh X of Lusignan, Count of La Marche, and was thus a half-brother to Henry III, and uncle to Edward I. William was born in the , Couhé-Vérac, Vienne, Poitou-Charentes, near Lusignan, sometime in the late 1220s (his elder sister Alice was born in 1224).