His parents were William IV "The Rich" Duke of Julich-Cleves-Berg(1516-92) and Maria of Austria (1531-81), a daughter of Ferdinand I, Holy Roman Emperor and Anna of Bohemia and Hungary. He grew up and was educated in Xanten. John William became Bishop of Münster. However, he was needed to succeed his father as Duke of Jülich-Cleves-Berg, a secular fief. He was also Count of Altena.
John William was first married in 1585 to Jakobea of Baden (d. 1597), daughter of Philibert, Margrave of Baden. He was secondly married to Antonia of Lorraine (d. 1610), daughter of Charles III, Duke of Lorraine. He had no known legitimate children.http://familytreemaker.genealogy.com/users/m/i/l/Glenn-W-Miller/GENE5-0001.html Jülich-Cleves-Berg was a combination of states of the Holy Roman Empire.
Upon Duke John William's childless death in 1609, his inheritance was claimed by the heirs of his two eldest sisters: the heir of Maria Eleonora of Cleves (1550-1608), the eldest sister, was Anna of Prussia, the Electress of Brandenburg, a Protestant. The second sister was Anna of Cleves (1552-1632), and her son and heir was the then Count Palatine of Neuburg, a Catholic. The disputes of the epoch between Protestants and catholics escalated, leading to the Thirty Years War in 1618; the succession dispute became part of the war. Ultimately, Brandenburg received Cleves-Mark and Neuburg received Julich-Berg, after the lands had been trampled under military several times and lost much of the fabled wealth so renowned in duke William IX's time.