He went to school in Kalmar and Växjö, enrolled at the universities of Greifswald in 1652 and studied there and at Wittenberg and Rostock until 1654.
Hahn has been characterized (Olsson, see below) as a man less suited to learning than to practical issues, and his main achievement was as leader of the Swedification of the provinces Skåne and Blekinge.
The realization of "uniformity" in church language, liturgy and education was a violation of the treaty but the war had convinced the Swedish authorities that it was necessary to push it through in order to retain the provinces.
Hahn called in an assembly of parish priests from the diocese in 1681 and convinced them to send a request to Governor-General Rutger von Ascheberg for the realization of uniformity throughout the provinces.
According to Hahn's biographer Bror Olsson, the Swedification went smoothly and was largely completed by his death in 1687.