Horst Janssen's mother, Martha Janssen, was a dressmaker from Oldenburg. He never knew his father. He was brought up in Oldenburg by his mother and grandparents in Lerchenstraße fourteen. His grandfather adopted him, and was later adopted by the Guardianship Court in 1939 after the death of his grandfather.
In 1942, he became a student at the Nationalpolitischen Erziehungsanstalt (Napola - National Political Education Institute) in Haselünne, Emsland, where an art teacher Hans Wienhausen encouraged his artistic talent. His mother died in 1943. In 1944 he moved to Hamburg, after being adopted by his mother's younger sister Anna Janssen, who lived in the city. He spent the end of the war and the post-war period with his Aunt Anna in Burchardstraße. They later moved to Warburgstraße (Harvestehude). In 1946 at the age of sixteen, Janssen enrolled at the Landeskunstschule (Regional Art School) in Hamburg. He was an outstanding pupil of Alfred Mahlau from the outset.
His first publication was a drawing in the weekly journal Die Zeit in 1947, followed next year by the publication of his first book, the children's book Der Wettlauf zwischen dem Hasen und dem Igel. In 1950 his first child, son Clemens, was born, and his second book, The race between the hare and the hedgehog, was published. Janssen produced his first woodcuts, influenced by Edvard Munch. Dominant themes were animals along with man and woman. In 1952 he received a Lichtwark scholarship in Hamburg. At around the same time, he was forced to leave the Landeskunstschule. The following year, he was arrested after a drunken brawl and tried for suspected murder. Janssen finally received a suspended sentence for drunkenness. His life was marked by the numerous marriages, outspoken opinions, alcoholism, and selfless dedication to the art of printmaking.
In 1954, he took up lithography and married Marie Knauer in 1955, who gave him his second child, a daughter named Katrin (nicknamed Lamme) in 1956. During this period, he worked hard on a series of large-scale color woodcuts that were displayed in his apartment in 1957. Janssen gained recognition and the same year had an exhibition in Hanover in the Hans Brockstedt Gallery. After this successful show, he suddenly switched to etching, becoming a pupil of Paul Wunderlich, whom he later considered to be a rival. In 1959, his marriage to Marie ended in divorce. His art was now influenced by art brut and Jean Dubuffet. A new marriage, to Birgit Sandner, was followed by separation a few weeks later. The following year, 1960, he married Verena von Bethmann Hollweg who, in 1961, gave birth to his third child, a son named Philip.
In 1964 Horst Janssen was awarded the Darmstadt Art Prize. In 1965, a retrospective of his drawings and graphic works appeared in the Kestner-Gesellschaft in Hanover. His position as a respected artist was bolstered by winning the Edwin-Scharff-Prize of the city of Hamburg in 1966. The following year, two of the most important people of his youth died: his Aunt Anna and his teacher Alfred Mahlau. Janssen moved to Blankenese, Mühlenberger Weg. The following year, he was divorced from his third wife. His art now dealt with the losses of these years. He won first prize for graphic art at the Venice Biennale in 1968. Janssen began a love affair with Gesche Tietjens. A trip with Gesche to Svanshall in southern Sweden lead to many beautiful drawings of the coastlines. His concentration on landscapes was supported by his return to etching. In 1972 he separated from Gesche Tietjen, who was pregnant with his child, Adam. In 1973 he had a love affair with Bettina Sartorius. In 1975 he won another prize, the Schiller Prize of the city of Mannheim, where his large drawing retrospective was organized in 1976. In 1990 the balcony of his house collapsed and he suffered injuries to his eyes. In 1992 the city of Oldenburg made him an Honorary Citizen.
Horst Janssen died on 31 August 1995 and was buried in the
Gertruden Cemetery in Oldenburg.