According to the Gesta Berengarii Imperatoris, William was a Frank who crossed the Alps leading 300 armed retainers in 888–889 to fight alongside Guy III of Spoleto against Berengar of Friuli for the Iron Crown of Lombardy.
He apparently established himself in northwestern Italy, probably supported by Guy, where he eventually received the title of comes.
It is also probable that he gave his support to Berengar after Guy's death, for he appears, in 921, along with Lambert, Archbishop of Milan, and two other counts, Giselbert and Samson, as dilectissimi fideles of the Emperor.
He appears for the last time alive in 924, intervening on behalf of the bishop of Piacenza with Rudolf II of Burgundy, a claimant for the Italian crown.
Various legendary assertions about his Saxon and Kentish origins and the origins of his wife have been met by the definitive Dizionario Biografico with the pronouncement: Ma tali asserzioni non sono ancora state seriamente coinprovate da documenti: "But such assertions are not yet seriously backed up by the documents."