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.
In 1150, William is introduced for the first time, witnessing the founding of his family's presence in Arles as a result of his father's agreement with the House of Baux after Barcelona's military takeover of Provence led by Raymond Berengar IV.