In 1834, he was employed as a teacher of religion, Norwegian, geography, and history at the Naval Cadets' School in Stavern.

[3] His strong interest in historical sources led him to Copenhagen to study the Arnamagnæan Manuscript Collection, and he published several minor works in his Samlinger til Det Norske Folks Sprog og Historie (Collections of Norwegian Popular Language and History; 1834–1837).

Lange belonged to the circle known as the "Norwegian historical school" that formed around Rudolf Keyser, Carl Richard Unger, and P. A.

Lange provided the initiative to start publishing the series Diplomatarium Norvegicum and he edited the first five volumes.

Lange was also the behind the series Norske rigs-registranter (Norwegian National Registers), a collection of legislation and decisions made by the Danish-Norwegian kings from 1523 to 1660,[4] but he died shortly before the first volume was published.