Williamstown was a large, triangular shaped region on the northeast side of the Seigniory of Beauharnois. It was bounded on the northwest by the Châteauguay and English Rivers, on the southwest by the English River and South Georgetown region, on the south by the Norton Creek and Edwardstown region and on the northeast by the Seigniories of Châteauguay and La Salle. It included the Villages of Ste-Martine and St-Urbain-Premier.