He participated in international championships over a period of more than ten years.

After setting a world record on the 1,500 meters with the time 2:02.5 on 5 February 1972, Magne Thomassen conquered the first position on the Adelskalenderen ranking for a period of two days until Kees Verkerk regained that position.

[3] He became the first winner of the Norwegian sprint speed skating championship in 1970, a title he also won the year thereafter.

[4] Thomassen won silver in the 500 m event of the 1968 Winter Olympic Games,[5] silver in the World Allround Speed Skating Championships in 1968 and 1970, bronze in the 1970 World Sprint Speed Skating Championships as well as bronze in the 1968 allround European Speed Skating Championships.

[5] Note that Thomassen's score on the Big combination was no world record because Kees Verkerk skated an even lower total of points on that day.