Stefan Sendłak was an activist in the Polish Socialist Party. He was arrested shortly after the German invasion of Zamość in 1939. Since he went on supporting the meanwhile banned party and was therefore at risk of arrest by the Gestapo, he fled to Warsaw in 1940. He continued his underground work there. In 1941 Sendłak began obtaining forged documents for Jews imprisoned in the camps around Zamość and in the ghetto. When the ghetto was liquidated in 1942, Sendłak supported Jews escaped from there, founding the Zamość-Lublin Committee for Aiding Jews. From the fall of 1943, Sendłak headed the Lublin section of the Żegota aid organization. He took care of around 300 people in hiding on their behalf. Sendłak took part in the Warsaw Uprising in 1944; after its suppression he was interned temporarily in transit camp 121 at Pruszków. He achieved his release by means of bribery.
Stefan Sendłak was honored as Righteous Among the Nations in 2020.