Larsen became the Danish national 5,000 and 10,000m champion in 1937, earning Helsingør IF's first running title. He went on to capture ten more national championships, nine in the marathon (1941, 1943–1950) and one in the 20 km (1950), as well as coming in third place in the 10,000m in 1938. In 1946 he set the Danish national record for the marathon at 2:38:36 and competed in that year's European Athletics Championships, placing eighth in the marathon. His next major international stop was the 1948 Summer Olympics, where he came in tenth in a field of 41 competitors in the same event.