Nielsen rode in the British League for the Oxford Cheetahs from 1984 to 1992. In 1986 he recorded a league average of 11.83, his average in away league matches that season was a perfect 12.00. With the Cheetahs, Nielsen won the British League Riders' Championship in 1986, 1987 and 1990, the British League Pairs Championship with Simon Wigg in 1985 and 1986 and in 1987 with Andy Grahame, the British League in 1985, 1986 and 1989, as well as winning the British League Knockout Cup in 1985 and 1986. He was also the 1984 "Pride of the Midlands" winner at the Norfolk Arena in King's Lynn.