While studying, she was a member of the Labour Party-affiliated Sosialistisk studentlag. However, she was excluded together with a group from this organization after visiting East Germany. Seip became a member of the Socialist People's Party at its foundation in 1961, and served for some time as a member of Bærummunicipal council.[1]
Seip was not hired in an academic position until 1974, when she became associate professor in criminology at the University of Oslo, covering for Tove Stang Dahl who was on a one-year leave. After one year, Seip was hired at the Institute of History.[1] She was promoted to professor in 1985.[2] Her field was modern Norwegian history, specifically social history, and her main works are Om velferdsstatens framvekst (1981), Sosialhjelpstaten blir til. Norsk sosialpolitikk fra 1740 til 1920 (1984) and Veien til velferdsstaten: norsk sosialpolitikk 1920-1975 (1994). She also penned volume eight of Aschehougs Norgeshistorie.[3]
Seip was married to historian Jens Arup Seip, whom she met as a tutor at the university. The couple had two children, Ingebjørg and Åsmund, both of whom are academics. Jens Arup Seip died in 1992. Seip still lives at Høvik in Bærum.[1]
