William was son of Ebalus Manzer and Emilienne. He was born in Poitiers. He claimed the Duchy of Aquitaine from his father's death, but the royal chancery did not recognise his ducal title until the year before his own death.
Shortly aftered the death of King Rudolph in 936, he was constrained to forfeit some land to Hugh the Great by Louis IV. He did it with grace, but his relationship with Hugh thenceforward deteriorated. In 950, Hugh was reconciled with Louis and granted the duchies of Burgundy and Aquitaine. He tried to conquer Aquitaine with Louis's assistance, but William defeated them. Lothair, Louis's successor, feared the power of William. In August 955 he joined Hugh to besiege Poitiers, which resisted successfully. William, however, gave battle and was routed.
After the death of Hugh, his son Hugh Capet was named duke of Aquitaine, but he never tried to take up his fief, as William reconciled with Lothair.
He was given the abbey of Saint-Hilaire-le-Grand, which remained in his house after his death. He also built a library in the palace of Poitiers.