After serving in World War I where he was severely wounded, Reichwein received his doctorate in 1920. Appointed Professor of History at the Halle Teachers College in 1930, the socialist philosopher and pedagogue was stripped of his position by the Nazis three years later.
From 1933 to 1939 he was obliged to earn his living as a village school teacher before being appointed Director of the Folklore Museum in Berlin. From 1941 Reichwein took part in the deliberations of the Kreisau Circle and was their link man with the Social Democratic wing of the Resistance. Together with Julius Leber, Reichwein undertook on 22 June 1944 to make contact with underground communist leaders in East Berlin, on behalf of Claus von Stauffenberg. One of the communists turned out to be a Gestapo stool pigeon and this led to Reichwein's arrest on 4 July 1944.
Tried before the People's Court and condemned to death on 20 October 1944, Reichwein was hanged on the same day.