From childhood until his public debut in 1962 his mother has been his piano teacher. He studied music theory and composition with Bj?rn Fongaard and Finn Mortensen. He visited the Internationale Ferienkurse in Darmstadt and attended a composition course in Stockholm.
Until 1961 he was mainly occupied with jazz, both as a pianist and as a composer and arranger.
Janson composes for both large and small ensembles, often integrating vocal elements. His early works show an interest in European avant-gardism, but many of his more recent works integrate many influences such as late romanticism, jazz and Norwegian folk music.
His music is often classified as neoexpressionism, making sonority a central element in many of his compositions, as, frequently is a political theme.