Neilson made his FIS Snowboard World Cup debut in December 1997 in Whistler . He won this competition and finished third in the Snowboard Cross World Cup at the end of the season. His second World Cup appearance was in Whistler the following year, which he finished in ninth place. At the beginning of the 2001/02 season, he won third place in Valle Nevado and Tignes . In the further course of the season he came under the top ten five times, including second place in Bad Gastein and Ruka and thus achieved second place in the Snowboard Cross World Cup. In the following season he took two wins in four World Cup appearances and thus finished fourth in the Snowboard Cross World Cup. He won the bronze medal at the season highlight of the 2003 Snowboard World Championships on Kreischberg . In the 2003/04 season he won his fourth World Cup victory in Jōetsu and finished third the next day. At Mount Bachelor he came second and at the end of the season he finished third both in the overall and in the snowboard cross world cup. At the beginning of the 2004/05 season he won in Valle Nevado. This was followed by two top ten placements and at the end of the season sixth place in the snowboard cross world cup. In the following season he reached nine World Cup appearances, six top ten placements, including second and one in Bad Gastein. At the 2006 Winter Olympics in Turin , he came in 17th place. He finished the season in eighth place in the overall world cup and in second place in the snowboard cross world cup. In the 2006/07 season he won three World Cups and took third place once. He won the snowboard cross world cup and achieved 11th place in the overall world cup. At the highlight of the season, the 2007 Snowboard World Championships in Arosa , he was tenth. After third place at the beginning of the 2007/08 season in Valle Nevado, he made it into the top ten four times. He achieved second place in Lake Placid and at the end of the season he finished 21st in the overall World Cup and fifth in the Snowboard Cross World Cup. In his last seasons of 2008/09 and 2009/10 he mostly achieved placements in the midfield. At the 2010 Winter Olympics in Vancouver , he finished eleventh.