Like most racers began Clarke his career in karting, where he was active from 1994 to 2000. He then moved to the British Formula Ford and decided the BRDC rating for themselves. In 2002, he competed in the Junior Evaluation of the British Formula Ford and was with two wins fourth. In 2003 he started in the regular championship. Initially he occupies the twelfth place overall, he remained in 2004 in the British Formula Ford and served with a victory behind Valle Mäkelä runner-up. He also took part in six races of the British Formula Renault.
2005 Clarke moved to the British Formula 3 Championship and launched as team-mate Bruno Senna for Raikkonen Robertson Racing. Clarke won a race and finished the season in fifth place ahead of Senna in the drivers' standings.
2006 wrote the Briton a contract for the Champ Car Series in CTE Racing HVM, where he was teammate of Nelson Philippe. While Philippe with a race win came in fourth overall, Clarke reached a podium placing third in Denver. In the overall standings, he was twelfth. He was also behind Will Power second- best rookie. 2007 was the racing driver in his team that ran as Minardi Team USA this season. As a teammate, he was the former Formula 1 driver Robert Doornbos. His best finish he scored in Elkhart Lake, where he finished the race in second place. In Zolder he was, after he had caused an accident with multiple race cars in training, do not participate in the race. Clarke was again not with his teammate, who finished third with two wins, keep up and finished at the end of the season 13th overall. After the reunification of the Champ Car to the IndyCar Series, the Briton was not a cockpit and joined 2008 to no race.
Beginning of 2009 took part in Clarke three race weekends of the A1GP series for the British team. He also hopes the next season again for the British team to compete, but the A1GP was dissolved after a bankruptcy and Clarke was again without cockpit. After Clarke had participated in Formula 2 testing in December 2009, he returned to North America and started for Walker Racing in the Indy Lights. He replaced Jonathan Summerton from the second race of the 2010 season. At the end of the season, he finished with two second places as the seventh best results overall.