 Arnim was born in the Prussian capital Berlin, the son of envoy Friedrich Abraham Wilhelm von Arnim (1767–1812) and his wife Georgine von Wallmoden-Grimborn (1770–1859), a daughter of the Hanoverian field marshal and art collector Johann Ludwig von Wallmoden-Gimborn and thereby presumably a granddaughter of King George II of Great Britain.His parents divorced, when he was three years old.Having finished his studies in Berlin and Göttingen in 1825, he joined the Prussian Guards Uhlans regiment as One-year volunteer and afterwards entered civil service at the Kammergericht.