Career
He represented the Østerbro-based club Københavns Idræts Forening throughout his career. Nielsen participated in three major championships - two Olympic Games and one European Championships. At the 1954 European Championships in Bern, Nielsen set a new Danish record of 3:44.4 in the final of the 1500 metres, in which he finished second behind Roger Bannister.
Nielsen was Denmark"s greatest male athlete during the 1950s and his ability to draw large crowds meant that he was much in demand by his country"s promoters.
His hectic schedule eventually took its toll on his health, and, with his times strongly affected by illness and exhaustion, Nielsen announced in September 1956 that he would retire after the 1956 Olympics in Melbourne. Following his athletic career Nielsen worked as a typographer for Det Berlingske Officin.