William was a younger maternal half-brother of Marie de Champagne and Alix of France. He was an older brother of Henry the Young King, Matilda of England, Richard I of England, Geoffrey II, Duke of Brittany, Leonora of Aquitaine , Joan Plantagenet and John of England.
At the time of his death, he had already been given the title of Count of Poitiers. For centuries, the dukes of Aquitaine had held this as one of their minor titles, so it had passed to Eleanor from her father; giving it to her son was effectively a revival of the title, separating it from the dukedom. Some authorities say he also held the title of "Archbishop of York", but this is probably an error. His half-brother Geoffrey Plantagenet (d. 1212), who was born within months of William, did later hold that office, causing the confusion.