Unlike many motor sports enthusiasts, Mardenborough has no extensive experience in karting . Although he rode on a kart track in Cardiff , where he grew up , for between eight and eleven years , his family did not have the financial means to continue his hobby of motorsport after the track was closed.
In 2011 he took part in the GT Academy , a competition organized by Nissan and Sony , which uses virtual car races on the PlayStation to determine a winner who enables them to enter professional motorsport. Mardenborough prevailed against 90,000 participants in this competition. As a title bonus, it was used by Nissan in the 24-hour race in Dubai . He achieved third place in his class. In 2012, Mardenborough received a GT3 cockpit in the British GT Championship. Together with Alex Buncombe he won a race and finished sixth in the GT3 classification. He also took part in four races in the Blancpain Endurance Series .
In 2013, Nissan decided to use Mardenborough in formula racing. First, he participated in the season of the Toyota Racing Series in part in New Zealand. As the best newcomer, he finished the season in tenth place. Mardenborough then received a cockpit from Carlin for the European Formula 3 Championship in 2013 . He was in the points four times and at the end of the season was the worst Carlin driver in 21st place overall. Mardenborough also drove for Carlin in the British Formula 3 Championship in 2013 . With a second place as the best result, he was sixth in this championship. In addition to his involvement in formula racing , Mardenborough also took part in the FIA World Endurance Championship (WEC) for Greaves Motorsport in the Le Mans 24-hour race in 2013 . Together with Michael Krumm and Lucas Ordoñez , he finished ninth and third in the LMP2 class. Mardenborough also came back to the Blancpain Endurance Series in 2013 and achieved a podium finish in its class in two participations.
In early 2014 Mardenborough took part again in the Toyota Racing Series and started for Giles Motorsport . He won three races and was on the podium a total of seven times. With 782 to 790 points, he was runner-up behind Andrew Tang at the end of the season . He then received a cockpit at Arden International for the season of the GP3 series . Mardenborough won a race and finished ninth overall as the best driver of his racing team. In addition, he competed again for OAK Racing in the 24 Hours of Le Mans and drove one race each in the United SportsCar Championship (USCC) and the British GT Championship. In 2015 Mardenborough switched to Carlin within the GP3 series. With two third places as the best results, he was ninth overall. When Carlin had a vacant cockpit at a 2015 GP2 series event , Mardenborough was used in the GP2 series instead of the GP3 . He also skipped the last race weekend of the GP3 series. In addition, Mardenborough received a factory cockpit from Nissan Motorsports for the 2015 FIA World Endurance Championship . The team only entered the series for the third race, the Le Mans 24-hour race , and then withdrew from the championship. In 2015, Mardenborough also took part in the VLN endurance championship Nürburgring . In the airfield section , his vehicle took off without any external interference and was catapulted over the safety fence when it hit the tire stack. One spectator was killed and two others were injured. Mardenborough himself survived the accident unharmed.