Karlsson played his first major tournament when he took part in the 1948 Winter Olympics in St. Moritz . There he was able to reach 11th place with jumps on 65.5 and 68 meters. This made him the most successful Swedish participant in this discipline. At the Swedish Championships in 1949 on the normal hill, he won his first and only national championship title. After the Heini-Klopfer ski jumping hill was opened in 1950 , he traveled frequently to Germany , as this was the first time he was able to jump over 100 meters.