Hallstein was a member of several nominally Nazi professional organizations, but he was not a member of the Nazi Party or of the SA. He is reputed to have rejected Nazi ideology and to have kept his distance from the Nazis. There was opposition from Nazi officials to his proposed appointment, in 1941, as professor of law at the University of Frankfurt, but the academics pushed through his candidacy, and he soon advanced to become dean of the faculty.
Hallstein began his academic career in the 1920s Weimar Republic and became Germany’s youngest law professor in 1930, at the age of 29. During World War II he served as a First Lieutenant in the German Army in France. Captured by American troops in 1944, he spent the rest of the war in a prisoner-of-war camp in the United States, where he organised a “camp university” for his fellow soldiers.