Geiger had originally joined the Braunschweig University of Technology (Brunswick) in 1924, progressing from being a visiting lecturer, to an associate professor, and finally becoming a full professor of sociology in 1929; this was the first professorship of the department for cultural studies. Geiger's work is still kept at the "Theodor Geiger Archive" at the university. He worked there until 1933, when, due to his anti-Nazi beliefs, he had to emigrate to Denmark; here he lived until 1943, even obtaining Danish citizenship. In 1939, he wrote "Sociologi", which was for several decades an important textbook.