He was a student of Christian Jakob Kraus (1753–1807) and of Albrecht von Thaer (1752–1828), whom he later described as an unforgettable teacher and patron ("unvergesslichen Lehrer und Gönner").

In Prussia he enjoyed a formidable reputation as a result of his writings and in 1809 he was recruited into the Prussian civil service, advancing to the grade of Regierungsrat.

According to a diary entry by his sister Florentine, Hagen's home along the "Sackheimer Tränkgasse" in Königsberg became a social hub for members of Prussia's political and scientific elite.

Hagen demanded, as urgent and necessary, national reform and the lifting of burdensome obligations owed by peasant farmers to the land owners.

[3] As early as 1814 Hagen was promoting the idea that the burden of indebtedness between peasant farmers and land owners could be ameliorated through the application of special reduced interest rates on outstanding debt burdens, to be applied using credit institutions established to mediate the debtor-creditor relationships, and through reforming land ownership laws.

His ideas anticipated measures that came to be implemented more than a generation, notably with the creation of the early Agricultural Credit Bank ("Landwirtschaftliche Rentenbank") founded in 1849.

His most important publications were: He also worked on the "Neuen preußischen Provinzialblättern" (newspaper) run by his brother Ernst August Hagen between 1846 and 1857.