Landgraviate of Hesse-Kassel
William was born in Kassel, the eldest son of Landgrave Philip the Magnanimous and Christine of Saxony. After his father's death in 1567, the Landgraviate of Hesse was divided between the four sons out of the late Landgrave of Hesse's first marriage, and William received the portion around the capital Kassel, the Landgraviate of Hesse-Kassel.