Leon Feiner, a lawyer, worked as an activist and journalist for the Jewish Labor Bund in Kraków. When the German Wehrmacht invaded Poland in 1939, Feiner fled the city. While crossing the border illegally, he was arrested by Soviet border guards. Shortly after the German-Soviet War began in 1941, Feiner managed to escape from prison in Lida (today Belarus). He reached German-occupied Warsaw, where he went underground and assumed a false identity to live outside the ghetto. Feiner joined Jewish resistance groups that were preparing an armed uprising. He gathered and circulated information about the mass murder of Jews. In his reports he called upon the Polish exile government in London to provide material aid to the persecuted Jews and to deliver weapons to those in the Warsaw ghetto. In December 1942, Feiner became vice chairman of the underground organization Żegota, and as of August 1944 its chairman. By then seriously ill with cancer, he remained in hiding in Warsaw, at times in the apartment of Eugenia Wąsowska-Leszczyńska, until Warsaw was liberated in January 1945. He died only a month later.