In the 1930s, Sendler conducted her social work as one of the activists connected to the Free Polish University. From 1935 to October 1943, she worked for the Department of Social Welfare and Public Health of the City of Warsaw. She also pursued informal, and during the war conspiratorial activities, such as rescuing Jews, primarily as part of the network of workers and volunteers from that department, mostly women. Sendler participated, with dozens of others, in smuggling Jewish children out of the Warsaw Ghetto and then provided them with false identity documents and shelter with willing Polish families or in orphanages and other care facilities, including Catholic nun convents, saving those children from the Holocaust.
The German occupiers suspected Sendler involvement in the Polish Underground and in October 1943 Sendler was arrested by the Gestapo, but she managed to hide the list of the names and locations of the rescued Jewish children, preventing this information from falling into the hands of the Gestapo. Withstanding torture and imprisonment, Sendler never revealed anything about her work or the location of the saved children. She was sentenced to death but narrowly escaped on the day of her scheduled execution, after Żegota bribed a German soldier to obtain her release.
In communist Poland Sendler continued her social activism but also pursued government career. In 1965, she was recognised by the State of Israel as Righteous Among the Nations. Among the many decorations Sendler received were the Gold Cross of Merit granted her in 1946 for the saving of Jews and the Order of the White Eagle, Poland's highest honour, awarded late in Sendler's life for her wartime humanitarian efforts.
Sendler was born Irena Krzyżanowska on 15 February 1910 in Warsaw, to Dr. Stanisław Krzyżanowski, a physician, and his wife, Janina. She grew up in Otwock, a town about 15 miles (24 km) southeast of Warsaw, where there was a vibrant Jewish community. Her father, a humanitarian who treated the very poor, including Jews, free of charge, died in February 1917 from typhus contracted from his patients. After his death, the Jewish community offered financial help for the widow and her daughter, though Janina Krzyżanowska declined their assistance.
From 1927, Sendler studied law for two years and then Polish literature at the University of Warsaw, interrupting her studies for several years from 1932 to 1937. She opposed the ghetto benches system practiced in the 1930s at many Polish institutions of higher learning (from 1937 at the University of Warsaw) and defaced the "non-Jewish" identification on her grade card. She reported having suffered from academic disciplinary measures because of her activities and reputation as a communist and philo-Semite. By the outbreak of World War II she submitted her magister degree thesis, but had not taken the final exams. Sendler joined the Union of Polish Democratic Youth (Związek Polskiej Młodzieży Demokratycznej) in 1928; during the war she became a member of the Polish Socialist Party (PPS). She was repeatedly refused employment in the Warsaw school system because of negative recommendations issued by the university, which ascribed radically leftist views to her.