From October 1979 until 1986 he worked for the Norwegian campaign against nuclear weapons, «Nei til atomvåpen», of which he was a co-founder.
His basically Gandhian nonviolent position was influenced by the Norwegian philosopher Arne Næss and peace researcher Johan Galtung.
[19] In 1974–79 he was a member of a public commission appointed by the Norwegian government to evaluate and propose changes in the legislation on conscription and conscientious objection in Norway («Vernepliktsutvalget»).
[2][21] He later was information officer and international secretary of the campaign,[22] which became one of the largest popular movements in post-war history in Norway.
[23] He was also one of the first Norwegian signatories of the European Nuclear Disarmament appeal[24][25] for a nuclear-free Europe and had close contacts to this movement.
Jon Grepstad studied English and French, Latin and Italian at universities in the United States, Norway and France.
In his teens he learned Esperanto, and in high school, German, in addition to English and French.