Printmaker, painter, and illustrator Horst Janssen was born on November 14, 1929 in Hamburg, Germany. He was first raised by his mother in Oldenburg, but having been born out of wedlock was soon adopted by his grandfather; he never knew his father. Janssen's exceptional gift for drawing showed became evident early on, and he would be encouraged in his artistic pursuits by his teachers throughout his school years.

After the death of his grandfather in 1939, Janssen became a ward of the Guardianship Court while continuing his studies, and he was enrolled in the boarding school of the National Political Institute of Education, an arm of the Nazi regime, in Emland in 1942. In 1943, after the death of his mother, his aunt Anna Janssen adopted him and took him out of the boarding school, bringing him to Hamburg where they waited out the rest of the war. He studied at the academy of fine arts of the Regional Art School (now HFBK Hamburg), where noted graphic artist and professor Alfred Mahlau recognized Janssen's talent and accepted him into his master printing class, becoming his mentor. This would prove to be the beginning of a successful career that would gain Janssen international fame.

His first published illustrations were in the weekly newspaper Die Zeit in 1947, and his first major retrospective took plae in 1965 at the Kestner Gesellschaft in Hanover. He was awarded Hamburg's Edwin Scharff Prize in 1966, the Grand Prize at the Venice Biennale in 1968, and was given an exhibition at the prestigious documenta VI in Kassel. He exhibited internationally, including in the cities of New York, Chicago, Los Angeles, Tokyo, Venice, Rome, Oslo, Moscow, Paris, Hamburg, and Oldenburg, among others.

Later in his life, Janssen's work focused on his childhood years in Oldenburg, and in 1992 he was made an honorary citizen of the City of Oldenburg. He died August on 31, 1995 in Hamburg and was buried according to his own will in the cemetery of St. Gertrude in Oldenburg.