The Nazis took her to the notorious Pawiak prison, which few people left alive. Gestapo agents tortured her repeatedly, leaving Sendler with scars on her body - but she refused to betray her team.
"I kept silent. I preferred to die than to reveal our activity," she was quoted as saying in Anna Mieszkowska's biography, "Mother of the Children of the Holocaust: The Story of Irena Sendler."
Zegota, an underground organization helping Jews, paid a bribe to German guards to free her from the prison. Under a different name, she continued her work.
After World War II, Sendler worked as a social welfare official and director of vocational schools, continuing to assist some of the children she rescued.
"A great person has died - a person with a great heart, with great organizational talents, a person who always stood on the side of the weak," Warsaw Ghetto survivor Marek Eldeman told TVN24 television.
In 1965, Sendler became one of the first so-called Righteous Gentiles honored by the Yad Vashem Holocaust memorial in Jerusalem for wartime heroics.
Yad Vashem Chairman Avner Shalev said Sender's "courageous activities rescuing Jews during the Holocaust serve as a beacon of light to the world, inspiring hope and restoring faith in the innate goodness of mankind."
Despite the Yad Vashem honor, Sendler was largely forgotten in her homeland until recent years. She came to the world's attention in 2000 when a group of schoolgirls from Uniontown, Kan., wrote a short play about her called "Life in a Jar."
It went on to garner international attention, and has been performed more than 200 times in the United States, Canada and Poland.
Sendler, born Irena Krzyzanowska, said she lived according to her physician father's teachings, arguing that "people can be only divided into good or bad; their race, religion, nationality don't matter."
She married Mieczyslaw Sendler but they divorced after the war's end. Sendler then married fellow underground activist Stefan Zgrzembski, and they had two sons and a daughter. One died a few days after birth. The second son, Adam, died of a heart failure in 1999.
Sendler is survived by her daughter and a granddaughter.