In 1934, he enrolled in the Warsaw Academy of Fine Arts and graduated in 1939. During World War II and the Nazi occupation of Poland, Tomaszewski earned a living through his painting, drawings, and woodcuts which were later destroyed during the Warsaw Uprising. In 1947, he began creating posters for state-run film distribution agency Central Wynajmu Filmow with fellow designers Tadeusz Trepkowski and Tadeusz Gronowski. Postwar shortages of supplies made Tomaszewski rework film posters and introduced bold colors, abstract shapes, and filmmaking techniques to convey the film's mood rather than rely on portraits of the film's stars. He also created posters for the circus and art exhibitions, among others. His poster designs were animated and witty, leading him to become the "father" of the Polish Poster School in the postwar era influencing international poster designers. His work was also part of the painting event in the art competition at the 1948 Summer Olympics.