 She worked as a teacher in the Academy of Fine Arts, Helsinki, for many years, and later trained graphic artists and wrote instructional books about graphic arts.Pietilä began her studies at Turku Drawing School (now TUAS Arts Academy of Turku University of Applied Sciences), which she attended from 1933 to 1936, and went on to study at the drawing school of the Finnish Art Association (1936–1940), the University College of Arts, Crafts and Design in Stockholm (1945–1949) and the Fernand Léger Art Academy in Paris (1949–1953).During her studies, she met her future life partner Tove Jansson; they collaborated on many works and projects, including the Moomin works, in collaboration with Pentti Eistola.