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 in 2002 where he remained for two seasons before moving to 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 bar. He attempted to relaunch his professional road career in the US when he signed for Continental Pro team 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. Without a contract but still motivated to ride he won the National Road Race title in 2006 and back to back Tour of Southland and Tour of Wellington titles in 2006 and 2007. In addition to the road races he returned to the track and won several titles at the New Zealand and Oceania track championships. Roulston had invested a six-figure sum with a New Zealand company that failed in October 2007 during the global financial crisis. Roulston confided in Craig Adair, a track cycling gold medallist at the 1982 Commonwealth Games, that he was about to pull out of the preparations for the 2008 Summer Olympics, but Adair and four of his friends decided to provide finance for him during this difficult time, and Roulston continued with his preparations. 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 and Team Pursuit 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 3000 m individual pursuit. At the 2008 Beijing Olympics, Roulston won the silver medal in the 4000 m individual pursuit, defeated in the gold medal race by Bradley Wiggins. He also featured as a member of the Bronze medal winning New Zealand team pursuit squad, although he did not race in the qualifying rounds. He came 10th in the Madison with his team mate Greg Henderson. In September 2008, Roulston announced that he would be riding for Cervélo TestTeam in 2009, with riders including reigning Tour de France champion Carlos Sastre and multiple Tour de France stage winner Thor Hushovd. In the 2009 Tour of California, Roulston began a perfectly executed lead out to allow team sprinter Thor Hushovd to win Stage 3. In stage 7 Roulston almost won the stage himself after breaking free from a ten-man breakaway that included Fränk Schleck, George Hincapie and Christian Vande Velde. Roulston was 2nd after a photo finish with Rinaldo Nocentini. Roulston finished 35th overall. Roulston came second in the Men's road race at the 2010 Commonwealth Games, winning the silver medal. 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.
