His Olympic speed skating career began with a rather inauspicious debut, as he failed to finish the 10,000m in 1948, the only event in which he competed that year.
Andersen triumphed in the three long races (1,500m, 5,000m, 10,000m) in his native Oslo. He won the 5,000m race with an astounding margin of 11 seconds and also won the 10,000m by a substantial margin.
Hjalmar competed at the 1956 Olympic Games, finishing 11th in the 5,000m and 6th in 10,000m, but then retired for good. His strength was as a distance skater, and he was world champion at 5,000m and 10,000m from 1950-52 consecutively. He set one world record at 5,000m and three at 10,000m.