An influential figure and one of the leading science administrators in Germany's nuclear energy project, Mentzel served as the scientific and technical adviser on the development of atomic bombs to the German government, and on some part, as the director of this program.

During this period, his sphere of responsibility also momentarily grew and he soon was appointed as the president of the German Association for the Support and Advancement of Scientific Research, acting as the director of the nuclear program.

After World War II, he was interned for three years by the American military government in post-1945 Germany, but was soon released after evidence proved no affiliation with the Nazi Party.

At Göttingen, he was a Kreisleiter (circuit leader) of the Nationalsozialistische Deutsche Arbeiterpartei (NSDAP, National Socialist Workers Party), of which he had been a member since 1922.

[14] Over the life of the Uranverein, even with the transfer of the RFR to Göring’s purview, Mentzel, as Ministerialdirektor of the REM, was able to make inputs to and exert some influence on the project.

His release was, in part, brought about by the affidavit provided by Walther Gerlach, at Mentzel’s request in a letter dated December 6, 1948.

Mentzel, as a science policy maker, had sided with the pragmatists in promoting scientific efficiency, rather than with the politically motivate party ideologues.