During the Second World War he was obliged to work as a Wehrmacht cartographer.
Since 1946 he ran a studio at
Bornheim near Bonn. 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 1951 he organized an exhibition for his friend,
Max Ernst. In 1955, 1959 and 1964, Fassbender participated at the
documentas I-III in
Kassel.
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.