Haakon Tønsager won bronze in the outriggers coxed fours at the Stockholm Olympic Games. The boat from the Christiana Roklub beat Austria (Leitmeritz) by abandonment in the first round and the Belgian boat from Gand in the second. In the semi-final, they narrowly lost to the eventual silver medallists from the Thames Rowing Club.
Tønsager graduated from the commercial school in 1909 and then went to Germany for one year. Subsequently, he worked for the graphic department of the E. C. Gjestvang Company as a salesman. From 1917, he spent some time in the United States before he joined the Grafisk Kompani A/S in Kristiania (now Oslo) in 1919.
He was married to Ingjerd Larsen and had one daughter. Haakon’s brother Ejnar coxed the Norwegian eight in 1908 and the outriggers coxed four in the 1912 Olympics.