Born to Ferdynand and Joanna of Sienkiewicz, Michalina spent her early life in her home village of Grodno, Congress Poland where she was a teacher in nature and geography until attending the University of Geneva.

[1][2] Stefanowska took a temporary job at the Solvay Institute of Physiology in Brussels and was joined there in 1898 by her colleague and former housemate from Geneva and Paris, Józefa Joteyko.

In 1908, she returned to Poland and held a lecture position for four years in physiology and neurology at one of the Wydz higher education facilities in Warsaw.

Throughout most of World War I and the German occupation of Poland (1912–1917), she headed the Orzeszkowa Gymnasium for girls while concurrently lecturing at the University of Poznan.

Her early research, published in the Solvay Institute's Travaux, dealt with nerve-cell organization, neurotransmission mechanisms and the effects of anesthesia and electrical stimulation on cortical circuits.