In 2009, aged only fifteen, King tested a Formula Two car. He drove four races in Formula Palmer Audi at the Silverstone round in 2010, managing to get a podium and impressing series boss Jonathan Palmer. In 2011, he has signed up for three rounds of the Formula Two championship,[4][5] at Spa-Francorchamps, the Nürburgring and Brands Hatch during the Formula Renault UK summer break. King will be the youngest driver to take part in the modern era of the series.
GP2 Series
In February 2015, it was confirmed King would move up to GP2, signing with Racing Engineering, alongside Alexander Rossi, for 2015, where he finished twelfth overall.[6] He remained with the team for the following season, where he claimed two victories and finished seventh overall.
For 2019, King returned with the same team partnered with Indian driver Mahaveer Raghunathan. King missed the Monaco round due to his participation in the 2019 Indianapolis 500, with Artem Markelov serving as his substitute. King scored two podium finishes during the season, claiming the 3rd position in the feature race at Baku and 2nd in the sprint race at Monza. He finished ninth in the standings.
Since 2020, King has been a development driver for the Alpine F1 Team.[10]
IndyCar Series
In 2018, King signed with Ed Carpenter Racing as a part-time driver in the No. 20 car, sharing driving duties with owner Ed Carpenter. Carpenter started the ovals, while King started the 11 road and street circuit races. In his IndyCar Series debut, the season opening Firestone Grand Prix of St. Petersburg, he set the track record in qualifying, made the Firestone Fast 6 and took the lead of the race on only the fifth lap. His second Fast 6 appearance came at the Indianapolis Motor Speedway during qualifications for the INDYCAR Grand Prix. He also paced the field during the only international stop in 2018, the Honda Indy Toronto. King advanced out of the first round of qualifications in half of the possible races over the course of the season.
King first tested Formula E machinery twice, firstly in 2016-17 with Mahindra Racing,[14] and again in a private test with Dragon Racing in 2019.[15] In February 2021, it was announced that King would be taking on a sim and development driver role at Mahindra.[14] In April 2023, King would participate in the Berlin rookies' driver test with the team.[16]