In 1153 he helped arrange the truce between Stephen and Henry Plantagenet, known as the Treaty of Wallingford, which brought an end to The Anarchy.

[2] When Henry Plantagenet ascended the throne as Henry II, he confirmed William's earldom and gave him direct possession of Arundel Castle (instead of the possession in right of his wife (died 1151) he had previously had).

In 1143, as Earl of Lincoln, he made two charters confirming a donation of land around Arundel in Sussex to the abbey of Affligem in Brabant, with William's brother, Olivier, present.

[3] William is the first proven English supporter of the crusader Order of St. Lazarus of Jerusalem and before 1146 had granted them land at Wymondham and built a Leper Hospital near his castle in Norfolk.

[4][dead link‍] His wife, Adeliza, was also a major benefactor to leper hospitals at Wilton, Wiltshire and Arundel[4][dead link‍] and his cousin, Roger de Mowbray and his family, were to become the most significant patrons of the Order's headquarters at Burton Lazars Hospital.