Ernest was a son of Duke Albert II of Brunswick-Wolfenbüttel-Göttingen and his wife, Rixa of Werle.
After his father's death in 1318, Ernest's older brother Otto the Mild took up government.
After Otto died childless in 1344, Ernest and his older brother Magnus I divided the Duchy.
It consisted at this time of the former County of Northeim, the cities of Göttingen, Uslar, Dransfeld, Munden and Gieselwerder and one half of Moringen.
In 1339, Ernest married Elizabeth, a daughter of Landgrave Henry II "the Iron" of Hesse.