He was born into an assimilated Jewish family, his mother ran a shop selling candy and haberdashery. His father, a tradesman, left the Jewish community during the time of the Austro-Hungarian monarchy. After Kárný graduated at the gymnasium, he continued his studies of history and Czech language at the Charles University of Prague between 1937–39. During this time, he became a member of the students' communist organisation Kostufra. Due to his Jewish origin, in November 1941 he was deported to the Theresienstadt ghetto. He was deported on 24 November 1941, with the first transport "Ak". In Theresienstadt he met his future wife, Margita Krausová (1923–1998). Margita and Miroslav were active in the Communist resistance group in Theresienstadt and they collaborated with Josef Taussig, Bruno Zwicker, Valtr Eisinger, Josef Stiassny and Friedl Dicker-Brandeis. In September 1944, they both were deported to Auschwitz. From here, Kárný was deported for slave labour to the auxiliary camp Kaufering of the Dachau concentration camp.
After the war, he became a journalist and then a freelance historian, specializing in the Holocaust and German fascism. He was expelled for the first time from the Communist Party (KSČ) due to condemnation of his brother Jiří in the anti-Semitic Slánský trial (1952), and for the second time in 1969, after the Warsaw Pact invasion of Czechoslovakia. He retired in 1973.
- "Miroslav Kárný o nasem case" (in Czech). www.terezinstudies.cz. 22 February 2002. Retrieved 15 December 2008.
- Berenbaum, Michael & Gutman, Yisrael (eds). Anatomy of the Auschwitz Death Camp. Indiana University Press and the United States Holocaust Memorial Museum (1994); this edition 1998. ISBN 0-253-20884-X
- Jiří Kárný was then working as a manager, closely with Ludvík Frejka, one of the main defendants. Frejka was hanged; Jiří received a long sentence. See "Miroslav Kárný (1919–2001)". By Raimund Kemper, p.6 (German).