William was the older brother of Theobald II, Count of Champagne; Stephen, King of England; and Henry, Bishop of Winchester.

Several historians have taken the view that he was passed over because of mental deficiency;[a] hence his soubriquet "William the Simple".

William was at first groomed to inherit the comital thrones of Blois and Chartres, and was designated count shortly before his father's departure on his second crusade in 1102.

[4] William was soon removed from wide-ranging comital duties by his mother, and when her second son Theobald came of age, around 1107, Adela elevated him to the position of Count of Blois-Chartres.

[5] On 13 November 1104,[6] William married Agnes of Sully, heiress to the lordship of Sully-sur-Loire,[7] chosen as his wife by his mother, Adela.