Mroszczak was one of the founders of Katowice's Free School of Painting and Drawing, where he ran a graphic arts studio in the years 1937–1939, at the same time conducting courses at the Institute of Commercial Education in Chorzów.
Mroszczak moved to Warsaw in 1952 and obtained the post of creative director at the publishing house Wydawnictwo Artystyczno-Graficzne (1952–1956) and a lectureship at the city's Academy of Fine Arts.
Mroszczak also lectured at numerous foreign universities, such as the Akademie für Bildende Kunst in Berlin-Weissensee (1958) and Hochschule der Künste in West Berlin (1964).
He designed pavilions at various international expositions and fairs, including Poznań, Paris, Milan, Barcelona, Leipzig, Stockholm, Helsinki, İzmir, Bari, Brussels, and Vienna.
He was also an honorary member of the Dutch Graphic Artists Association (GVN) and of the Academia Nazionale de Belli Arte in Parma.
PRL" ("Polish Artistic Endeavour after 15 years of the People's Republic of Poland"), Warsaw (Auschwitz never again project) (Work, Happiness, Peace, Freedom, Progress)