Courtney was World Junior Karting Champion in 1995 and world Formula A Champion in 1997. He won the British Formula Ford Championship in 2000, setting a record for most wins in a single season. A drive with Jaguar's Formula Three team in 2001 was followed by a test driver role with the Jaguar Formula One team.
In 2002, while testing at Monza, the rear suspension on Courtney's Jaguar F1 car failed at high speed, sending the car into the barrier at 306 km/h and subjecting him to an estimated 67G impact. Michael Schumacher, testing for Ferrari at the same circuit, was among those who pulled Courtney from the wreck. Courtney could not see for weeks after the accident and required a year of recovery, suffering from severe headaches and light sensitivity. He described the experience as the moment he resolved never to fear crashing again.
Unable to continue his F3 championship push after missing races during recovery, Courtney moved to Japan. He won the 2003 All-Japan Formula Three Championship for the TOM'S team and raced in the All Japan Grand Touring Car Championship (Super GT) for Toyota in 2004 and 2005.
Courtney made his V8 Supercars debut in the 2005 endurance rounds with the Holden Racing Team alongside Jim Richards. He joined Stone Brothers Racing in 2006 as replacement for two-time champion Marcos Ambrose. He finished third at the 2006 Bathurst 1000 on debut and second at the 2007 Bathurst 1000, establishing himself as a frontrunner. His first race win came at the 2008 City of Ipswich 400 at Queensland Raceway.
Courtney moved to Dick Johnson Racing in 2009 with a new Triple Eight Race Engineering-built Ford FG Falcon. After an accident-marred 2009 season he won at Townsville and Sydney, finishing seventh. In 2010, he converted championship potential into the title, winning five races across Queensland Raceway, Winton, and Sandown, taking the championship ahead of reigning champion Jamie Whincup.
Courtney joined Walkinshaw Racing (then the Holden Racing Team) for 2011, where his career trajectory stalled despite a race win at the 2011 Yas V8 400 at Abu Dhabi. He remained with the team through multiple ownership changes as it became Walkinshaw Andretti United, but consistent race wins proved elusive. A severe accident in 2013 at Phillip Island, when Alexandre Prémat's car made heavy contact with Courtney's door with enough force to snap the safety bars, left Courtney with a torn quad muscle and a fractured tibia and kept him out of the final four races of the season. A further incident in 2015 — when a low-flying Navy helicopter blew a piece of metal signage into the pit lane, striking Courtney and causing two broken ribs and a punctured lung — cost him further racing time.
Courtney continued to compete in Supercars in later years primarily in a co-driver capacity, pairing with David Reynolds at Team 18.
Courtney's 2010 championship title is the centrepiece of his racing career and one of the few V8 Supercars titles won by the Dick Johnson Racing team. His path to Australian touring cars — via European karting championships, a near-fatal F1 test crash at Monza, and a Japanese Formula Three title — is among the more unusual biographical trajectories in the Supercars paddock.