Radio and electronics engineer. One of the first graduates of the Warsaw University of Technology (WUT) in 1919 and the Military Academy for communications officers in Paris (1922). As such, he was a lecturer on radio engineering in military schools.

In 1928 he received a doctorate degree for his thesis on radio wave stability compensation method (Metoda kompensacyjna kontroli stałości fali) and was appointed Chair of Radio Engineering at the WUT Faculty of Electrical Engineering. In 1929 he was appointed associate professor and subsequently, professor in 1935.

In 1928 he initiated the establishment of the first radio engineering institute in the country (later Institute of Telecommunications), which he directed since 1934. During occupation years, he was the underground lecturer at University. Simultaneously to his educational career, he was a soldier of the Polish Home Army, in which he managed to decipher missile navigating systems of V-2 rockets. In 1945 he founded the WUT Faculty of Electrical Engineering (later Faculty of Communications), where he chaired the Radiolocation Institute until retirement in 1968. He organized the first Polish Academy of Sciences (PAN) research units: the Electronics Unit and The Institute of Fundamental Technological Research in Science (IPPT), to which the unit was later incorporated. In 1961-1963 he was appointed Director of IPPT PAN.

Janusz Groszkowiski was an outstanding expert in high vacuum measurement. He gained international recognition for his work on ionizing heads for low-pressure measurement. Author of 16 patents and 361 scientific papers, also books (Techinika wysokiej próżni, 1972). Supervisor of 33 dissertations.

Since 1936, he was a member of Akademia Nauk Technicznych (Academy of Technical Sciences), since 1949 – Towarzystwo Naukowe Warszawskie (Warsaw Scientific Society), since 1952 he was a member of the PAN, since 1957 – its vice-president, and then in 1962-1971 its president. He resigned from being a member of the Sejm (lower house of the Polish Parliament) of the PRL - People’s Republic of Poland, and vice-president of Rada Państwa (the State Council), as an act of protest against progressing political repression in 1976. Member of 6 foreign academies of sciences (Czechoslovakia, Hungary, the Soviet Union, Romani, Bulgaria and Cuba) and honorary member of many associations. Laureate of the First Class State Award (1951, 1955, 1968, 1979). He was conferred the degree of honorary doctor (doctor honoris causa), at: Warsaw, Gdansk, and Łódź Universities of Technology. Holder of state awards: the Gold Cross of the Virtuti Militari War Order, Commander's Cross of the Order of Polonia Restituta (twice), Officer's Cross of the Order of Polonia Restituta.

The Military Communication Institute credited professor Groszkowski’s scientific achievements, giving the Institute his name.