A native of Helsinki, Jansson was the son of a sculptor, Viktor Jansson, and a Swedish-born illustrator, Signe Hammarsten-Jansson. His siblings included an older sister, writer Tove Jansson, and an older brother, photographer Per Olov Jansson. In 1958, he began working with his sister on the writing of the Moomin comic strip, which he drew from 1961 to 1974. Between 1990 and 1992, Jansson worked with Dennis Livson to develop the concept of the Moomin animated series in Japan. His daughter, Sophia, worked together with him in 1993 to help manage the production of a new series of Moomin strips which Sophia now manages solely.