Guillaume d'Ercuis was the almoner to King Philip III of France and the tutor of his son, King Philip IV. He was a royal notary, and, as one of the King's men, a canon of the cathedrals of Laon, Noyon, Senlis, Mello, Marchais and of Reims, archdeacon of Laon and of Thiérache. He derived his name from his small seigneurie of Ercuis in the Beauvaisis between Neuilly-en-Thelle and Cires-lès-Mello, about 55 km from Paris. At Ercuis he erected a chapel dedicated to Virgin Mary, served by the monks of the Abbey of St Genevieve, Paris.