County of Mark
William was born in and died in Düsseldorf. He was the only son of John III, Duke of Jülich-Cleves-Berg, and Maria, Duchess of Jülich-Berg. William took over rule of his father's estates (the Duchy of Cleves and the County of Mark) upon his death in 1539. Despite his mother having lived until 1543, William also became the Duke of Berg and Jülich and the Count of Ravensberg.