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. Originally a National Socialist by political orientation, Mentzel served as one of the top leading science policy-makers to Adolf Hitler and his cabinet in his role as an undersecretary of the Reich Ministry of Education (REM) in the Office for Science. In the Kaiser Wilhelm Society, he was on the advisory board and, during World War II, Second Vice President of the Kaiser Wilhelm, supervising the clandestine research critical to develop the atomic bombs.