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.