Background
Ernest was the eldest son of Duke Philip I of Brunswick-Grubenhagen (1476–1551) and his second wife, Countess Catherine of Mansfeld-Vorderort (1501–1535), eldest daughter of Count Ernest II and his first wife Barbara of Querfurt. Together with his father and brothers, he took part in a military campaign in southern Germany in 1546, during the Schmalkaldic War, which ended unsuccessfully at Ingolstadt.