In late Medieval times, noblemen were generally known for the most important fief that they held.

He was not present in the September 1345 campaign against the Frisians, which saw William IV killed during the disastrous Battle of Warns.

[2] William of Duivenvoorde and his nephew John II were leaders of what would become known as the 'Hook' party during the Hook and Cod wars.

In 1350, he purchased the Land of Breda for 43000 florins from John III, Duke of Brabant.

Oud Haerlem Castle was taken after a siege which lasted more than 6 months, even though John van not present.

Whatever the later events in Holland, John seems to have concentrated his efforts on extending his holdings in the Breda area.

They had three children: In 1353, he married Matilda (c. 1324 – 1366), an illegitimate daughter of John III, Duke of Brabant.

They had two sons: In 1370, he married Margaret, a daughter of Otto, Lord of Lippe and Irmgard of the Marck.