Nielsen participated in three major championships - two Olympic Games and one European Championships. He made his international breakthrough at the 1952 Summer Olympics in Helsinki, where he finished fourth in the 800 metres - finishing with exactly the same time, 1:49.7, as the bronze medal winner, Heinz Ulzheimer. 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.