Hardenberg was the eldest son of Christian Ludwig von Hardenberg (1700-1781), a Hanoverian colonel, later to become field marshal and commander-in-chief of the Hanoverian army under king George III from 1776 until his death. The mother was Anna Sophia Ehrengart von Bülow. He was born, one of 8 children, at Essenrode Manor near Hanover, his maternal grandfather's estate. The ancestral home of the "knights of Hardenberg" is Hardenberg Castle at Nörten-Hardenberg, which the family acquired in 1287 and owns to this day. They were created barons and, in 1778, counts.