He was the second son of William I of Jülich and Joanna of Hainaut. He quarreled greatly with his father and imprisoned him from 1349-1351. When his father died in 1361, William became the second Duke of Jülich, his elder brother Gerhard having predeceased their father.
William was co-ruler from 1343. He tried for many years in Holland-Zeeland to enforce claims against the House of Wittelsbach but failed. He led the traditional feuds of the House of Jülich and lost Kaiserswerth and Zülpich, among others
William intervened in favor of Edward in the catastrophic war of succession between his brothers-in-law Reinald and Edward for control of the Duchy of Guelders.
He took part in the 1371 Battle of Baesweiler (in which Edward was mortally wounded) and captured Wenceslaus I, Duke of Luxembourg. In 1377, Emperor Charles IV granted Guelders to William"s son William, but the title was not secured until 1379.
Among other territories, William acquired Monschau-Montjoie, Randerath and Linnich.
