Born in Drammen, Karlsen was already a child with an unusual athleticism, and his mother recorded him in photos posing to show muscles at the age of three.

After finishing second in the Húsafell Stone carry, Karlsen was leading the finals on points, but during the warm-up for the squat event, he tore his hamstring and had to withdraw from the rest of the contest.

Karlsen returned to 1999 World's Strongest Man in Valletta, Malta, finishing in third place behind Jouko Ahola and Janne Virtanen.

The following year he promoted himself to second place in 2000 World's Strongest Man held in Sun City, South Africa behind Janne Virtanen.

[3] Karlsen retired from competition in 2006, and began serving as presenter and producer of the Norwegian version of the World's Strongest Man contest and the qualifying tour Giants Live.

Karlsen also ran the Viking Power Challenge event in Norway, which was a part of the Super Series and later Giants Live tour for qualification into the World's Strongest Man competition.