He was born in Oslo as a son of professor of medicine Jan Birger Jansen (1898–1984) and Helene Sofie Schøning (1902–1976).
degree with the thesis Afferent impulses to the cerebellar hemispheres from the cerebral cortex and certain subcortical nuclei.
In 1959 he started studies at the University Laboratory of Physiology in Oxford.
He won the Andres Jahre Prize for Young Researchers together with Per Andersen in 1967.
He was a member of the Norwegian Academy of Science and Letters from 1977, and chairman of the Nansen Fund.