an illegitimate son of Henry II of England, and a prominent baron, soldier, and administrator under John and Henry III. He acquired his lands and title from Richard I, who in 1196 gave him the hand of the heiress Ela, or Isabel, daughter of William, earl of Salisbury. He held numerous official positions in England under John.

He was sent on missions to France (1202) and to Germany (1209). In 1213–14 he organized John's Flemish allies, taking part in the destruction (1213) of the French fleet at Damme, then the port of Bruges, and leading the right wing of the allied army at Bouvines (July 27, 1214), where he was captured. He was exchanged and was back in England by May 1215, when he was employed by John in inspecting the defenses of royal castles and fighting the rebels in the southwest.

During John's war against the barons, Salisbury deserted the king after the landing of Louis of France (May 1216); he returned to royal allegiance, however, by March 1217, fought at Lincoln (May) and Sandwich (August), and attested the Treaty of Kingston (September 1217). Salisbury held various posts during the minority of Henry III and served against the Welsh in 1223 and in Gascony in 1225. He and his wife were benefactors of Salisbury Cathedral and laid foundation stones of the new cathedral in 1220. William was buried there and his effigy, a splendid early example, still survives.