Assisted by some two dozen other Żegota members, Sendler smuggled approximately 2,500 Jewish children out of the Warsaw Ghetto and then provided them with false identity documents and shelter, outside the Ghetto, saving those children from the Holocaust. With the exception of diplomats who issued visas to help Jews flee Nazi-occupied Europe, Sendler saved more Jews than any other individual during the Holocaust.
The German occupiers eventually discovered her activities, and she was arrested by the Gestapo, tortured and sentenced to death, but managed to evade execution and survive the war. In 1965, Sendler was recognised by the State of Israel as Righteous among the Nations. Late in life, she was awarded the Order of the White Eagle, Poland's highest honour, 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, who was a humanitarian treating the very poor free of charge, including Jews, died in February 1917 from typhus contracted from his patients. After his death, the Jewish community leaders offered to help her mother pay for Sendler's education, though her mother declined their assistance. Sendler studied Polish literature at Warsaw University, and joined the Polish Socialist Party. She opposed the ghetto-bench system that existed at some pre-war Polish universities and defaced her grade card. As a result of this public protest, she was suspended from the University of Warsaw for three years.
She married Mieczysław Sendler in 1931, but they divorced in 1947. She then married Stefan Zgrzembski, a Jewish friend from her university days, by whom she had three children, Janina, Andrzej (who died in infancy), and Adam (who died of heart failure in 1999). In 1959 she divorced Zgrzembski and remarried her first husband, Mieczysław Sendler. They eventually divorced again.
Sendler moved to Warsaw prior to the outbreak of World War II, and worked for municipal Social Welfare departments. She began aiding Jews soon after the German invasion in 1939, by leading a group of co-workers who created more than 3,000 false documents to help Jewish families. This work was done at huge risk, as—since October 1941—giving any kind of assistance to Jews in German-occupied Poland was punishable by death, not just for the person who was providing the help but also for their entire family or household. Poland was the only country in German-occupied Europe in which such a death penalty was applied.