During World War I the family lived mainly in Paris and traveled to Brittany and Lake Geneva. Romana, like other Polish women, actively supported Poles fighting in the Russian army on the Eastern Front, as mentioned by Karol's daughter Camilla in her biography. In November 1918 Karol took part in an exhibition organized to the benefits of the invalids of the Polish Army in France, together with 38 other artists. In the early 1920s, Konstanty Brandel came from newly independent Poland with a job offer at the "State School of Art Industry" in Bydgoszcz. Back to Poland Bydgoszcz On April 1, 1922 Mondral returned to Poland and settled in Bydgoszcz, working as the head of the Graphic Department at the National School of Arts and Crafts. The family lived at 11 Świętej Trójcy street and then in a villa at 3 Chopina street. At the school, Karol set up a vocational school for printers. Furthermore, he accepted a commissioning of the City Council for making etchings of views of Bydgoszcz: hence he documented the municipal architecture with around 100 engravings. Poznań and Warsaw In March 1, 1931 the painter was hired as a professor at the Faculty of Graphic Arts of the State School of Decorative Arts () in Poznań. Karol and his family only moved in September 1932, changing apartments several times. He eventually chaired the school studio of lithography and dry-engraving techniques until the outbreak of World War II. The start of the conflict found Mondral in Warsaw, on his way from Krzemieniec to Poznań. In the Polish capital, he received a sentence from the Nazi authorities to move to a resettlement camp in Poznań, then to a transit camp near Lublin. Back anew in Warsaw, he left the city at the outbreak of the Warsaw uprising in 1944, ending in Tarnów. During the occupation, the family's flat in Poznań was occupied by German soldiers: most of the furnishings and the works were looted (160 oil paintings, 200 plates of copper engravings, etchings, aquatints, about 60 woodcuts, about 130 copies, sculptures and valuable works by other artists). In 1945 he returned to Poznań and became a lecturer of graphic sciences at the State Higher School of Fine Arts. He retired in 1950. On the occasion of the 50th anniversary of artistic work, he received a touching gift - a graphic portfolio made by his students, dated June 28,