Flieg, born to a Jewish merchant family in Chemnitz, was an antifascist from an early age. In 1931, he was, at the instigation of local Nazis, expelled from the Gymnasium in his home town because of an anti-military poem. He completed school in Berlin, and began a degree in media studies there. After the 1933 Reichstag fire, he fled to Czechoslovakia, where he took the name Stefan Heym.