His father was Robert, 1st Earl of Gloucester, an illegitimate son of King Henry I of England, thus William was a nephew of the Empress Maud and a cousin of King Stephen, the principal combatants of the English Anarchy period.
In October 1141, William looked after the Baronial estates, when his father fell into the hands of partisans at Winchester. His father was exchanged for King Stephen, and during his father's absence in Normandy in 1144 he served as Governor of Wareham. In 1147, he overthrew Henry de Tracy at Castle Cary.
FitzRobert granted Neath, a town in Glamorgan, a charter. He was Lord of the manor of Glamorgan, as well as Caerleon, residing chiefly at Cardiff Castle. It was there that in 1158 he and his wife and son were captured by the Welsh Lord of Senghenydd, Ifor Bach ("Ivor the Little") and carried away into the woods, where they were held as prisoners until the Earl redressed Ivor's grievances.
Relationship with King Henry IIEdit
In 1173 the earl took the King's part against his sons, but thereafter he appears to have fallen under suspicion, for the following year he submitted to the King, and in 1175 surrendered to him Bristol Castle. Because his only son and heir Robert died in 1166, Earl William made John, the younger son of King Henry II, heir to his earldom, in conformity with the King's promise that John should marry one of the Earl's daughters, if the Church would allow it, they being related in the third degree.
Earl William was present in March 1177 when the King arbitrated between the Kings of Castile and Navarre, and in 1178, he witnessed Henry's charter to Waltham Abbey. But during the King's struggles with his sons, when he imprisoned a number of magnates of whose loyalty he was doubtful, Earl William was among them.