William was the son of Geoffrey I Villehardouin. In 1236 he aided the Latin Empire against the Byzantine Empire of Nicaea, and was rewarded with the overlordship of the Venetian Duchy of the Archipelago and other Venetian territories in the Aegean Sea. In 1239 he married the daughter of Narjot de Toucy and of Narjot's first wife (who was the daughter of the dowager empress Anna). William came to power in Achaea in 1246 when his brother Geoffrey II Villehardouin died.