At the call of Pope Urban II, William took up the cross of the First Crusade in the army of Raymond IV of Toulouse.
When William returned, he found that the Aimoin brothers to whom he had confided the administration of the lordship in his absence had usurped many seigniorial rights and that he was obligated to recognise much of their newfound authority, which diminished his own, in order to retain his position.
William participated in the army of Raymond Berengar III of Barcelona which captured Majorca from the Moors in 1114.
The rest of his reign was marked by the important acquisition of nearby territories, which greatly recouped his power: Montarnaud, Cournonsec, Montferrier, Frontignan, Aumelas, Montbazin, Popian.
By his marriage to Ermensenda, daughter of Peter, Count of Mauguio, he had six children: Upon his death, he was succeeded by his son William as Lord of Montpellier.