She died late Sunday at a Helsinki hospital.
Immonen was known for her one-of-a-kind outfits, celebrity clients and a question-and-answer column which she wrote for Eeva magazine in the 1950s and '60s.
A self-taught clothing designer, Immonen opened her first shop in Helsinki during the war, which grew into a great success despite the shortages of the ensuing years.
In 1951, Immonen co-founded Marimekko with her friend Armi Ratia. However after the firm's first season, she concentrated on her own Cinderella collections.
Last spring Immonen was the subject of an exhibition at Helsinki's Design Museum and of a biography by Ritva Koskennurmi-Sivonen.