Janosch was born as Horst Eckert on 11 March 1931 in what was then Hindenburg (now Zabrze, Poland) in Upper Silesia to a family of mixed German and Polish origin, though he identifies his nationality as "Silesian". His mother was a pious Catholic, and his father a violent alcoholic.

After World War II, the family fled to West Germany. In the area of Oldenburg, Janosch worked in a textile mill. In 1953, he went to Munich, where he studied for some terms at the Academy of Arts, but broke off his studies eventually. Afterwards, he worked as a freelance artist. In 1960, his first book for children was published with George Lentz, a friend of his, who also persuaded him to take on the penname Janosch.

By 1980, he had published over 100 books for children in nearly 30 languages. Amongst his most recognisable characters is the Tigerente that appears in Oh, wie schön ist Panama and the stories around the little tiger and the little bear. He has also published several books for adults, in which he deals with his childhood experiences of growing up with an abusive father and a God-fearing mother, constantly recurring topics being friendship, family relationships, and the quest for the meaning of life.