[8][15] 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.

[18][19][20] 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.

[15] Sendler became associated with social and educational units of the Free Polish University (Wolna Wszechnica Polska), where she met and was influenced by activists from the illegal Communist Party of Poland.

At Wszechnica Sendler belonged to a group of social workers led by Professor Helena Radlińska; a dozen or more women from that circle would later engage in rescuing Jews.

[8][19] Around four hundred thousand Jews were crowded into a small portion of the city designated as the Warsaw Ghetto and the Nazis sealed the area in November 1940.

[28] Sendler and other social workers would eventually help the Jews who escaped or arrange for smuggling out babies and small children from the ghetto using various means available.

[33] 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.

Sendler was known there by her conspiratorial pseudonym Klara and among her duties were searching for places to stay, issuing fake documents and being a liaison, guiding activists to clandestine meetings.

She described a commemoration event there, on the anniversary of the October Revolution but in the spirit of the Polish leftist tradition; it included artistic performances by children.

Initial funds for transfer and maintenance of ghetto children were provided by members of the Jewish community, still in existence, in cooperation with women from the Welfare Department.

Turkow, who contacted Wanda Wyrobek and Sendler to take out of the ghetto and arrange care for his daughter Margarita, wanted to prioritize children of the most "deserving" (accomplished) people.

She made desperate attempts to save her friends, but among her former Welfare Department associates unable or unwilling to leave the ghetto were Ewa Rechtman and Ala Gołąb-Grynberg.

According to Jadwiga Piotrowska, who saved numerous Jewish children,[38] during the Great Action people from the Welfare Department operated individually (had no organization or leader).

Other accounts suggest that women from that group concentrated on making arrangements for Jews who had already left the ghetto, and that Sendler in particular took care of adults and adolescents.

[42] In regard to the action of saving Jewish children, according to a 1975 opinion written by Sendler's former Welfare Department co-workers, she was the most active and organizationally gifted of participants.

[37] During the Warsaw Ghetto Uprising, a network of emergency shelters was created by Sendler's group: private residencies where Jews could be temporarily housed, while Żegota worked on producing documents and finding longer-term locations for them.

[17][31][50] Sendler was freed due to the efforts of Maria Palester, a fellow Welfare Department activist, who obtained the necessary funds from Żegota chief Julian Grobelny; she used her contacts and a teenage daughter to transfer the bribe money.

[50] On 30 November, Warsaw's mayor Julian Kulski asked the German authorities for permission to re-employ Sendler in the Welfare Department with back-pay for the period of her imprisonment.

She hitchhiked in military trucks to Lublin, to obtain funding from the communist government established there, and then helped Maria Palester to reorganize the hospital as the Warsaw's Children Home.

[55] Sendler and her co-workers gathered all of the records with the names and locations of the hidden Jewish children and gave them to their Żegota colleague Adolf Berman and his staff at the Central Committee of Polish Jews.

[59] According to the research done by Anna Bikont, in 1947 Sendler advanced to the party executive by becoming a member of the Social Welfare Section at the Central Committee's Social-Vocational Department.

[60][61] Especially prior to 1950, Sendler was heavily involved in Central Committee work and party activism, which included implementation of social rules and propagation of ideas dictated by the Stalinist doctrine, and policy enforcement; by engaging in such pursuits, she abandoned some of her previously held views and lost some important acquaintances.

[60][62] After the fall of communism, however, Sendler claimed having been brutally interrogated in 1949 by the Ministry of Public Security, accused of hiding among her employees politically active former members of the Home Army (AK), a resistance organization loyal during the war to the Polish government-in-exile.

[57] Materials dealing with her activities during the war were published,[27][62] but Sendler became a well-known public personality only after being "rediscovered" by the group from an American high school in 2000 (at the age of ninety).

[68] In the spring of 1967, suffering from a variety of health problems, including a heart condition and anxiety disorder, Sendler applied for a disability pension.

[98] In 2016, a permanent exhibit was established to honor Sendler's life at the Lowell Milken Center for Unsung Heroes museum, in Fort Scott, KS.

[99] Gal Gadot has been cast to play Sendler in a historic thriller written by Justine Juel Gillmer and produced by Pilot Wave.

W ukryciu ('Sendler: In Hiding'), a biography and book about the people and events related to Sendler's wartime activities, was written by Anna Bikont and published in 2017.

The Social Welfare Department of the Central Committee of Polish Jews stated in January 1947 that Sendler saved at least several dozen Jewish children.

[108] For the first time she talked about the list and the 2,500 saved children (and adults) in 1979; back then, however, she did not suggest that she was personally responsible for their survival and named twenty-four people also involved in their rescue.