Kofler was born of Jewish parents on 26 April 1907 in Chotymyr, Galicia, Austria-Hungary (now Ivano-Frankivsk Raion, Ivano-Frankivsk Oblast, Ukraine). World War I in 1915/16 drove his family to escape to Vienna, where Leo attended business school, until 1927. His working career was cut short by the 1929 stock crash, and he became an adviser of a social-democratic education center in Vienna, joining the left-wing of the social-democratic labour party (SDAP). From 1933-34, he devoted himself to research with Max Adler. In July 1938, after the annexation of Austria by Nazi Germany, he escaped to Basel, Switzerland where he was interned in an immigrant camp. Most of his family died in the Holocaust, and his parents were shot in 1942. Nevertheless, he continued his theoretical studies, being influenced especially by the writings of Georg Lukács. In 1944 he published his first book under the pseudonym "Stanislaw Warynski".