During his minority, his Greek possessions were governed by his illegitimate elder brother Alfonso Frederick. In 1330, Alfonso returned from Greece and William ceded the counties of Malta and Gozo to him. Nicola Lanza replaced him in Greece.
In 1337, his father willed him the Principality of Taranto, the county of Calatafimi, the honour of Monte Sant'Angelo, and various castles and lands in Noto, Spaccaforno, Capo Passero, and Avola when his mother, Eleanor, daughter of Charles II of Naples, died. She died in 1341, but William died first on 22 August 1338. He left his library to the Dominicans of Palermo and was buried in the cathedral there.