Roulston was a talented junior rider on both road and track and competed for New Zealand on the track and initially on the road for a club team in France. He turned professional with the French team Cofidis in 2002 where he remained for two seasons before moving to Discovery Channel for 2005. His season with Discovery Channel featured some impressive rides but was interrupted by injury and eventually ended when he resigned after an incident in a Christchurch Barometer
He attempted to relaunch his professional road career in the United States when he signed for Continental Pro team Health Net–Maxxis and began strongly with two top ten stage finishes in the Tour of California beating many big name ProTour regulars.
Unfortunately for Roulston his first year at HealthNet ended when a medical examination revealed irregular heart activity and he was advised to stop riding immediately. Back home in New Zealand he experimented with some alternative remedies and was soon back riding – and winning.
Roulston was selected for the New Zealand team to compete at the 2008 World Track Championships in Manchester where he narrowly missed medals in the 4000 m Individual Pursuit (4th) and Team Pursuit (4th) as well as finishing 9th in the Madison with Greg Henderson. Having performed so strongly in Manchester, he was selected for the Beijing Olympics, where he focused on improving his 4th placed pursuit rides.
Former New Zealand track coach Ron Cheatley suggested his best bet will be to drop the Madison and focus on the complementary pursuit events in much the same fashion as Kiwi pursuit rider Sarah Ulmer.
Ulmer quit road racing and focused solely on her pursuit preparation before the 2004 Athens Olympics where she went on to smash the world record and take the gold medal in the women"s 4000 m Individual Pursuit. He came 10th in the Madison with his team mate Greg Henderson. Roulston was 2nd after a photo finish with Rinaldo Nocentini.
Roulston finished 35th overall.
In October 2015 Roulston announced his retirement from road racing, having previously revealed that he would make a return to the track with a view to competing in the team pursuit at the 2016 Summer Olympics.