Janssen was a student of Alfred Mahlau at the Landeskunstschule Hamburg. He first published in the newsweekly Die Zeit in 1947. In the early 1950s, he started working in lithography, on an initiative of Aschaffenburg paper manufacturer Guido Dessauer, using the technical facilities of a coloured paper factory. The first retrospective of Janssen's drawings and graphic works was shown in 1965, first in the kestnergesellschaft Hanover, then in other German cities and in Basel. In 1966, he was awarded Hamburg's Edwin Scharff Prize. International exhibitions followed. In 1968, he received the Grand Prize in graphic art at the Venice Biennale; in 1977, his works were shown at the documenta VI in Kassel.
The Horst Janssen Museum in his hometown of Oldenburg is dedicated to his legacy. His work is shown internationally in major museums.
His life was marked by numerous marriages, outspoken opinions, alcoholism, and selfless dedication to the art of printmaking.Wikipedia