Nielsen was born in Arentsminde near Brovst, Denmark. He was one of, if not the most successful riders the sport has ever seen. Altogether he participated in winning 22 World Championships for Denmark (out of the 46 won by Danish riders/teams in all competitions to the end of 2014). This included the individual World Championship in 1986, 1987 (the only time the final was held over two days), 1989 and the first ever championship ran in the Grand Prix format in 1995. Nielsen and Sweden's Tony Rickardsson are the only 2 riders to have won world titles in both the old single meeting format and the Speedway Grand Prix series.