William inherited the Marquisate after the death of his elder brother John IV.

William served as condottiero for Francesco I Sforza of Milan and fought alongside him at the battle of Caravaggio(1448) and was afterwards rewarded with the lordship of Alessandria.

The reason was said to because he had formed an attachment to Sforza's wife Bianca Maria Visconti.

William married on 19 January 1465, Marie de Foix (d.1467), daughter of Gaston IV, Count of Foix;[3] and secondly on 18 July 1469, Elisabetta Sforza (d.1473),[4] daughter of Francesco I Duke of Milan and Bianca Maria Visconti;[5] and finally, on 6 January 1474, Bernarde de Brosse daughter of Jean II de Brosse.

[6] His second marriage to Elisabetta Sforza, they had: When William died in Casale Monferrato, he was succeeded by his brother Boniface III.