William first appears in 1150, witnessing his father's settlement at Arles with the House of Baux after Raymond Berengar IV of Barcelona's invasion of Provence.
They elected Theobald of the neighbouring County of Bigorre as their viscount, but, as he did not respect the Fors de Bearn, they executed him within the year.
They then selected a nobleman of the Auvergne, Sentonge, who lasted two years in power before suffering the same sad fate at the hands of the nobles.
In 1173, the year Sentonge was executed, Mary abandoned William with their two young twin sons and entered the monastery of Santa Cruz de Volvestre.
Setting this traditional, probably legendary account, aside, it is likely that William's eldest son Gaston VI inherited Béarn on Mary's death in 1173.