He attended the University of Angers and, while he was young, his family negotiated with the duke of Brittany to grant him the Bishopric of Vannes or Saint-Brieuc.
After Margaret de Clisson and her sons failed in their plot to overthrow John V, Duke of Brittany and assert their claim to Brittany between 1420 and 1422, the Penthièvre family was forced to appear before both the French Parliament and the Breton Estates at Vannes.
Two of William's brothers were found guilty of felonies and treachery on 16 February 1425 and their properties in Brittany were confiscated.
William remained a hostage at Auray for twenty-eight years, during which time he lost his sight.
William inherited the County of Périgord from his brother John in 1454, but he died the following year.