At intervals from 1793 to 1801 Lang was closely connected with the Prussian statesman Hardenberg, who employed him as his private secretary and archivist, and in 1 797 he was present with Hardenberg at the congress of Rastadt as secretary to the legation.
He was also archivist and then librarian of the city of Frankfort.
On the 14th of August 1789 the Constituent Assembly made Camus its archivist, and in that capacity he organized the national archives, classified the papers of the different assemblies of the Revolution and drew up analytical tables of the procesverbaux.
Motley acknowledges his indebtedness to Groen's Archives in the preface to his Rise of the Dutch Republic, at a time when the American historian had not yet made the acquaintance of King William's archivist, and also bore emphatic testimony to Groen's worth as a writer of history in the correspondence published after his death.
In 1855 he was appointed archivist at Breslau; in 1862 he became professor of history at Heidelberg, and ten years later professor at Berlin, where he was a member of the directing body of the Monumenta and a member of the Academy.