English: Grave of Jan Karski and Pola Nirenska-Karski at Mount Olivet Cemetery in Washington, D.C., in the United States.

Jan Karski was born Jan Kozielewski on June 24, 1914, in Łódź, Poland. He was raised Roman Catholic in a pluralistic neighborhood that included a substantial Jewish population. A graduate of a prestigious mounted artillery officers' school as well as the state-run diplomatic corps school, he started work in the Polish Ministry of Foreign Affairs on January 1, 1939. During the Nazi German invasion of Poland in September 1939, Karski's military unit was attached to the Kraków Cavalry Brigade, where he tried to defend the area between Zabkowice and Częstochowa. After Poland surrendered, he tried to escape to Hungary but was captured by the Red Army (the Army of the Union of Soviet Socialist Republics). He was interred in a prison camp in Ukraine, concealed his identity as an officer, and was turned him over to the Germans. (By concealing his identity, he escaped the Katyn massacre).

Karski escaped, returned to to Warsaw, joined the ZWZ (the Polish resistance movement), and adopted the nom de guerre "Jan Karski". He was arrested in Czechoslovakia in July 1940 and severely tortured by the Gestapo. The Polish resistance kidnapped him and smuggled him back to Warsaw. Karski was twice smuggled into the Warsaw Ghetto and once almost penetrated the Bełżec death camp to obtain first-hand evidence of the Holocaust. In 1942, Karski escaped to the United Kingdom with microfilm containing documentation about the Holocaust. His documentation, and his first-hand accounts, provided the Allies with one of the earliest and most accurate accounts of the Holocaust. He later met with President Franklin D. Roosevelt to discuss the issue.

After the war, Karski obtained a Ph.D. from Georgetown University in 1952, and was naturalized as an American citizen in 1954. He taught at Georgetown University for 40 years. Following the fall of communism in Poland in 1989, the democratic Polish government awarded Karski the Order of the White Eagle (the highest Polish civil decoration) and the Order Virtuti Militari (the highest military decoration awarded for bravery in combat).

In 1965, Karski married the 54-year-old dancer and choreographer Pola Nireńska. She was a Polish Jew whose family (with the exception of her parents) died in the Holocaust. She committed suicide in 1992. The couple had no children.

On June 2, 1982, Yad Vashem recognised Jan Karski as Righteous Among the Nations. Shortly before his death, he was nominated for the Nobel Peace Prize and formally recognized by the United Nations General Assembly. On April 23, 2012, President Barack Obama bestowed on Karski America's highest civilian honor, the Presidential Medal of Freedom.

Karski died of heart and kidney disease in Washington, D.C., on July 13, 2000. He was interred at Mount Olivet Cemetery in the same grave as his wife.