His father was the Prussian chamberlain (Kammerherr) Joachim Erdmann von Arnim (1741–1804), royal envoy in Copenhagen and Dresden, later active as the director of the Berlin Court Opera.

Arnim and his elder brother Carl Otto spent their childhood with their maternal grandmother Marie Elisabeth von Labes, the widow of Michael Gabriel Fredersdorf from her first marriage, in Zernikow and in Berlin, where he attended the Joachimsthal Gymnasium.

From 1800 he continued his studies at the University of Göttingen, though, having met Johann Wolfgang von Goethe and Clemens Brentano, he inclined from natural sciences towards literature.

Back in Germany, he began forming a collection of these and in 1805 first published the result, in collaboration with Clemens Brentano, under the title Des Knaben Wunderhorn.

He and Brentano completed the second and third volume of their folk song collection and from 1808 together with Joseph Görres published the important romantic Zeitung für Einsiedler (Newspaper for Hermits) in Heidelberg in 1808.

In Berlin, Achim worked on Heinrich von Kleist's legacy and founded the patriotic Deutsche Tischgesellschaft association of Christian men.

[2] He remained connected with the Prussian patriots such as Adam Heinrich Müller and Friedrich de la Motte Fouqué and even commanded a Landsturm battalion during the German Campaign of 1813.