An earlier Sir William le Brun, probably his ancestor, had come over with the Anglo-Norman invasion of Ireland.
Fromund Le Brun, Lord Chancellor of Ireland 1270–1283, was his cousin.
He was in holy orders and held a number of livings, including Montgomery and Dungarvan, and had custody of the See of Durham in 1240.
He was in the service of the prime royal favourite Peter des Rivaux, who held office briefly as Lord High Treasurer in the early 1230s.
Rivaux fell from power in 1234: thereafter William entered the Crown's service, and held a position in the Exchequer.