He specialized in the longest track distances, 5000 and 10,000 metres, eventually competing at the 1964 Olympic Games.
[2] He won the 10,000 metres in 1963 ahead of reigning champion Magnar Lundemo, and then took a bronze medal in 1964.
With his club IL i BUL he broke IK Tjalve's record in September 1958 at Bislett stadion with the time of 15:53.4 minutes.
[9] Unlike his brother who became a professor of history, Benum studied medicine in Oslo and Bergen.
From 1982 he was a professor of medicine at the St. Olav Hospital, connected to the Norwegian University of Science and Technology.
Here he headed the Orthopedics Department for many years, and for his achievements he was decorated as a Knight, First Class of the Order of St. Olav in 2006.