Kammala wannan jumlar: William shi ne ɗan fari na Duke na Brunswick Henry the Mild daga aurensa na farko da Sophia, 'yar Griffin Duke Wartislaw IV na Pomerania. Bayan mutuwar mahaifinsa a 1416, ya gaji Gidan Brunswick na Lüneburg wanda ya yi mulki tare da ɗan'uwansa ɗan'uwansa Henry mai zaman lafiya. William ya zama mai mulki mai ƙarfi; ba da daɗewa ba ya shiga rikice-rikice da yawa tare da sarakunan da ke makwabtaka da su kamar Archbishop na Bremen da Bishop na Hildesheim, kuma ya goyi bayan Count na Schauenburg da Holstein a cikin yaƙinsu da Sarki Eric na Denmark. Ya kuma yi yaƙi tare da Margrave Frederick na Meissen ya kayar da
Sojojin Hussite a cikin Yaƙin Brüx na 1421.