Mandel gained his abitur from Gymnasium Adolfinum Moers [de] in 1901, and subsequently studied theology at the universities of Halle, Königsberg, Bonn and Greifswald.
During this years, Mandel increasingly drifted away from traditional Protestant teaching, and became interested in the relationship between religion and race and ethnicity.
He saw the intellectual world as being divided into Western and Near Eastern components, and argued in favor of purging Christianity of all Jewish influences.
He joined Jakob Wilhelm Hauer's German Faith Movement and became a co-editor of its journal Deutscher Glaube.
He also became a member of the National Socialist Teachers League, the Nazi Party, the National Socialist German Lecturers League, and during World War II he participated in Aktion Ritterbusch [de].