Mathisen profited from his World War II physical activity. He took his first Norwegian national title in 1946, and eight more titles would follow, the last in 1953. Five of the titles came in relay races. He competed in the 1948 ICF Canoe Sprint World Championships, and received a silver medal in K-1 4 x 500 metres relay, together with teammates Ivar Iversen, Hans Martin Gulbrandsen and Eivind Skabo. Before the 1948 Summer Olympics, Mathisen and his teammate Knut Østby (also from Bærum) were considered as having the highest medal hopes in the Norwegian canoeing squad. Mathisen and Østby proceeded to win the silver medal in the K-2 10000 m event and finished fourth in the K-2 1000 m. Mathisen then received a silver medal in K-2 1000 m at the 1950 ICF Canoe Sprint World Championships, with Knut Østby. At the 1952 Summer Olympics in Helsinki, he finished fifth in both the K-2 1000 m and K-2 10000 m events.