Harald Nielsen played 14 games for the Denmark national team in 1959 and 1960, scoring 15 goals, and he was known as Guld-Harald (Gold-Harald).

From his position of center forward, Nielsen finished the top scorer of the league as Frederikshavn won promotion to the Danish football championship.

At Bologna, il freddo danese (the cold Dane) was a part of the 1963–64 Serie A winning squad, and he was the Italian league topscorer in both 1963 and 1964.

Following six seasons at Bologna, Harald Nielsen moved to Inter in 1967, in a transfer deal which made him the most expensive player in the world at the time.

Although he started by winning the unofficial world cup for teams by beating Santos with Pelé 1–0 in New York, Nielsen did not find the same degree of success with Inter as in Bologna, and following years at S.S.C.

Just after ending his career he started a firm together with his wife, former actress Rudi Nielsen, which imports Italian leather works to Scandinavia.

He was Protektor for the Legends Club and until his death watched all the home matches of FC København from Parken's A-stand.

[5] On 26 October 2016, which would have been his 75th birthday, the square in front of Arena North, in Frederikshavn, was named the Harald Nielsens Plads in his honor.