Fleszarowa was appointed in 1935 to serve as a Senator by the President of the Polish Republic. During her tenure, she focused on formal organization of scientific pursuits and expanding the rights of citizens. In 1937 was one of the organizers of the Democratic Club of Warsaw, serving as its vice-president and Democratic Alliance Party. Her Senate term ended in 1938 and during the Occupation of Poland, she joined the underground movement, participating in the Home Army. Working in developing information and propaganda for the Home Army, she distributed maps and published literature, setting up an editorial office for underground writings. She worked to hide Jews, and led secret meeting of librarians, leading the effort to hide the archive of the Association of Polish Librarians. Fleeing the city before the Warsaw Uprising in 1944, Fleszarowa went to Lublin and worked as a contact to the Polish Committee of National Liberation. In October 1944, she was appointed head of the Library Department of the Ministry of Education.