After the deaths of his elder brothers, John (killed at Kortrijk in 1302) and Henry (d. 1303), he became heir apparent to his father's counties.

Guy and Duke John II of Brabant then conquered most of Zeeland and Holland, but these territories were recovered again when William became the new count in the same year.

At the death of his uncle, Guy d'Avesnes, Bishop of Utrecht, William took his fief of Amsterdam and annexed it to Holland.

[3] With these important alliances William gained considerable influence and respect, which he used to advance the interests of his counties.

[2] On 19 May 1305 William married Joan of Valois,[1] sister of the future king Philip VI of France,[4] and had the following children: