Participating at the 1908 Summer Olympics, Larsen placed fourth of seven in his initial 1500 metres semifinal heat and did not advance to the final. At the 1912 Summer Olympics he participated in both 800 and 1500 metres, without reaching the final. He became Norwegian champion in the 800 metres in 1911 and 1914 and in the 1500 metres in 1911, 1914 and 1915.