He was an active member of the Deutsche Liga fur Menschenrechte (League for Men's Rights), a spinoff of the pacifist Bund Neues Vaterland, until 1933 when he fled for Sweden when the group was dissolved by Nazis.
He obtained his doctorate at Kiel in 1911 and after completing his habilitation, became a professor at the University of Hamburg, teaching German literature and Scandinavian studies.
Besides his academic career, he lectured and officiated at baptisms and weddings for the Hamburg Free Religious Community (freireligiösen Gemeinde) and the German Monist League.
In 1933, the Nazis seized power and introduced the legislation known as the Berufsbeamtengesetz, which, among other things, dismissed all Jews, including Berendsohn, from their government jobs.
He escaped imminent arrest by immigrating to Denmark, where he was assisted by a stipend from the American Guild for German Cultural Freedom in 1938–40, and again to Sweden during the rescue of the Danish Jews.