He participated in the Third Crusade under Emperor Frederick I alongside his father, Floris, and his uncle Otto of Bentheim.

On his way to the Egypt, he commanded a joined force of Frisian, Dutch, Flemish and Rhenish crusaders that helped the Portuguese conquered the city of Alcácer do Sal from the Almohads as it is noted in the De itinere Frisonum and the Gesta crucigerorum Rhenanorum.

Huge infrastructural works were done; the island called Grote Waard was enclosed with dikes all around and a dam was built at Spaarndam.

New governmental bodies were created, the so-called water boards, which were charged with the task of protecting the polders against ever-present threat of flooding.

Count William granted city rights to Geertruidenberg in 1213, to Dordrecht in 1217, to Middelburg in 1220 and perhaps also to Leiden.

In 1220 Count William married Marie of Brabant, widow of Emperor Otto IV.