He was the son of Nilon III, the Count of Bar-sur-Seine. In 1210, he assisted at the coronation of Jean de Brienne as King of Jerusalem. In 1211, he arbitrated between Leo I of Armenia and the Templars, regarding the castle of Bagras. During his rule, the order flourished in Spain, achieving important victories against the Moors. He had contact with the Mongols under Genghis Khan and was accused of treason as a result. Guillaume died in 1219 of pestilence, (possibly endemic typhus), as a consequence of being wounded during the siege of Damietta, in Seborga in the Holy Land.