He performed his military service at the prestigious Russian language program of the Norwegian Armed Forces.

He was later educated in history as well as Nordic linguistics and literature at the University of Oslo and achieved a cand.philol.

From 1988 to 1998 he served as director of the Centre for Technology and Culture at the university and from 1999 until his death in 2015 he was a fellow of the Institute for Social Research.

Sejersted was a member of the board at the Institute for Comparative Cultural Studies 1974–1982 and at the Norwegian Research Council for Basic Research 1980-1983 as well as chairman of the Norwegian Science Policy Council 1984–1988.

Sejersted suffered a long illness and died on 25 August 2015 in Oslo, aged 79.