William was the eldest son of the Brunswick duke Henry the Mild from his first marriage with Sophia, daughter of the Griffin duke Wartislaw IV of Pomerania. Upon his father's death in 1416, he inherited the Brunswick Principality of Lüneburg which he ruled jointly with his younger half-brother Henry the Peaceful. William turned out to be an energetic ruler; he soon entered into numerous feuds with neighbouring princes such as the Archbishop of Bremen and the Bishop of Hildesheim, and supported the Counts of Schauenburg and Holstein in their fight against King Eric of Denmark. He also fought with Margrave Frederick of Meissen defeating the Hussite forces in the 1421 Battle of Brüx.