Joseph Gregor studied musicology and philosophy at Vienna University, graduating in 1911. He worked under Max Reinhardt as assistant director and from 1912-1914 as a lecturer in music at the Franz-Josephs-University of Chernivtsi. He was employed at the Austrian National Library in Vienna in 1918. There he founded the Theater Collection in 1922, in which he included film after 1929. He also taught from 1932–1938 and 1943–1945 at the Max-Reinhardt-Seminar for actors, being granted the title "Professor" in 1933. Gregor wrote several illustrated pamphlets like the one published in 1930 and titled: Wiens letzte große Theaterzeit ("Vienna's Last Great Theater Epoch"). It was volume 12 in the series Denkmäler des Theaters ("Theater Monuments") which depicted famous set designs, theater artists, and costumes. He retired from his position at the National Library in 1953.

Gregor's role in the era of National Socialism has been disputed controversially: Gregor incorporated many libraries of politically persecuted intellectuals into the Austrian National Library. Some say he did so in order to save these libraries, others asserted that Gregor profited from the political persecution. 

Gregor became one of the main theater scholars in the service of the Third Reich, even though Fascist authorities were reserved about him due to his editorial performance at the magazine Die Theater der Welt ("Theaters of the World") in 1936-1937. On the other hand, his non-fiction works and also the libretto to the Strauss opera Friedenstag were popular among ranking Nazis.