Liebermann was born in the Galician town of Drohobych (today the Ukraine). He studied law in Vienna, Zurich, Paris and Kraków, and began his involvement with socialist policies while still at school. Between 1901 and 1919, he was an active member of the Polish Socialist Party (PPS). In addition, from 1907 to 1911 he was a member of the House of Deputies of the Austrian Imperial Diet, where inter-alia he campaigned against the frequent ill-treatment during soldier training. During the First World War, he fought as a simple soldier in the Polish Legion before being promoted to first lieutenant.