He was born in 1934 in Kraków as the son of Andrzej Rieger, a prosecutor and lieutenant of the Polish Army military reserve force, and Antonina Latinik, the first champion of Poland in woman's foil; a grandson of Roman Rieger, mining engineer, lecturer and inventor, and Franciszek Latinik, general of the Polish Army.

In his youth, between the 1940s and 1950s, Janusz Rieger was a scout and served as an altar boy at St. Florian's Church in Kraków, where Karol Wojtyła was a vicar.

Rieger was a member of the Catholic student group that rejected Marxist ideology and was the narrow circle of Wojtyła's pupils, the so-called Environment.

He was the organizer of the Polish Studies College for young scientists from the East at the Institute of Slavic Studies of the Polish Academy of Sciences (1993–1996, Stefan Batory Foundation grant) and the initiator of the International School of Humanities at the Center for Research on Ancient Tradition in Poland and Central and Eastern Europe of the University of Warsaw.

From 1988, he cooperated in publishing the Carpathian Dialectological Atlas as the chairman of the Polish team and co-editor of all volumes He was also a member of its editorial board (1997–1998).

[2] He edited and annotated his father's diary, which begins with enlistment in September 1939 and runs through Soviet captivity until his death in April 1940.

[1] By the decision of the President of Poland on 22 November 2017 "for outstanding contribution to the development of Polish Slavic studies, for achievements in scientific and didactic work and for popularizing the history and culture of Kresy", he was awarded the Knight's Cross of the Order of Polonia Restituta.