He was the third son of Marquess John Jacob, and inherited the Marquisate after the death of his elder brother John IV. He obtained by Emperor Frederick III the territories lost to Savoy from 1435. William served as condottiero for Francesco I Sforza of Milan and was later the tutor of the latter's son Galeazzo Maria. When the latter was assassinated, William acted as moderator in the Duchy of Milan.
William married thrice, but all the times without legitimate son. He thenceforth had his daughter Giovanna married to Ludovico I of Saluzzo.