Johnsen competed in his first ski jumping World Cup on March 21, 1987 on his home hill in Oslo. With 15th place he was able to win a World Cup point. After a mixed four hills tournament in 1987/88 , Johnsen stood on January 20, 1988 in St. Moritz, Switzerland, for the first time with a 2nd place on the podium at a World Cup competition.
Shortly after playing, he won the gold medal on the large hill at the Norwegian Championships in Vang . At his first World Cup competition in Lahti , he was able to jump onto the podium with third place in both competitions, before winning his first World Cup competition a week later on March 18, 1988 in Meldal . He repeated this success on the Holmenkollbakken in Oslo . It was the only two World Cup victories in his career. After a 12th place in the World Cup in Planica , he finished the 1987/88 season in 7th place in the overall World Cup ranking.
For the 1989/90 World Cup season , Johnsen no longer competed. In the 1990/91 season he played two more World Cup competitions, but remained unsuccessful and without points, so that he ended his active ski jumping career after jumping on his home hill in Oslo.