During the 1920s Fassbender was trained in painting by Richard Seewald at the Kölner Werkschulen.
During the Second World War he was obliged to work as a Wehrmacht cartographer.
In 1947 Fassbender founded at Schloss Alfter near Bornheim, together with Hann Trier, Hubert Berke and some other artists and men of letters, the "Donnerstagsgesellschaft" in order to revive, and promote, modern painting in the Rhineland.
In 1958 the artist was appointed professor of painting and drawing at the Kunstakademie Düsseldorf, where he retired in 1968.
[2] Fassbender also designed many placards and book jackets.