William was the eldest son and heir of Count Floris IV of Holland and Matilda of Brabant.

He gained a certain amount of theoretical support from some of the German princes after his marriage to Elizabeth, daughter of the Welf duke Otto of Brunswick-Lüneburg, on 25 January 1252.

The Count Palatine Otto II, who was also Duke of Bavaria, was excluded from taking part in the election on the ground that, as a supporter of Conrad IV, he was under sentence of excommunication.

"[4] In his home county, William fought with Countess Margaret II of Flanders for control of Zeeland.

In July 1253, he defeated the Flemish army at Westkapelle (in modern-day Belgium) and a year later a pause in hostilities followed.

[1] In battle near Hoogwoud on 28 January 1256, William tried to traverse a frozen lake by himself, because he was lost, but his horse fell through the ice.