Born and raised in London to a Scottish father and South Korean mother, Aitken began competitive kart racing aged seven.

He competed in the 2010 Super 1 National Kart Championships in the Rotax Mini Max class, with ProTrain racing alongside Jack Hawksworth, finishing third behind champion and future Formula 2 teammate George Russell.

He won the final race at Rockingham Motor Speedway and finished second in the standings behind Fortec Motorsport teammate Seb Morris.

Aitken also appeared as a guest driver in three rounds of the main Eurocup Formula Renault 2.0 series, recording a best finish of ninth place at the Red Bull Ring.

He also made guest appearances in the Formula Renault 2.0 Alps series, with best finishes of fourth place at the Imola Circuit.

He took nine race wins in total, finishing ahead of Louis Delétraz in the Eurocup and Jake Hughes in the Alps series.

In his maiden season, he claimed a victory at Spa-Francorchamps and six additional podiums to finish fifth in the overall standings, one point behind teammate Jake Dennis.

In February 2017, it was announced that Aitken would remain in the GP3 Series, switching to reigning team champions ART Grand Prix.

For the 2019 season Aitken moved to Campos Racing, initially partnering Dorian Boccolacci and later Arjun Maini and Marino Sato.

Aitken left Campos and the Formula 2 series after 2020, however he returned during the 2021 season with HWA Racelab as a replacement for Matteo Nannini, who withdrew from the championship after the first round.

[26] Aitken made his Formula One debut as a race driver with Williams at the 2020 Sakhir Grand Prix as a replacement for George Russell, who was promoted to Mercedes following Lewis Hamilton's positive coronavirus test.

[35] He did not partake in any practice sessions that year, and at the start of 2023, he was announced that he would split with Williams, in order to focus on his sportscar career.

[36] After leaving Campos in Formula 2, Aitken switched to sports car racing and joined the GT World Challenge Europe.

He competes in both the Endurance and Sprint cups, driving a Lamborghini Huracán GT3 Evo for Emil Frey Racing alongside Konsta Lappalainen.

He crashed into the barrier after Raidillon, bounced back onto the track and came to a stop before being hit from behind by another driver which caused two further cars to become entangled in the accident.

[2] In 2022, Aitken competed in the LMP2 Pro-Am class for Racing Team Turkey alongside Charlie Eastwood and Salih Yoluç.

Aitken made his 24 Hours of Le Mans in the 2022 edition, competing for Algarve Pro Racing alongside Sophia Flörsch and John Falb.

He won the 12 Hours of Sebring March 18 after the GTP cars in front of him crashed into each other with only minutes left in the race.

[1][41] In August 2023, it was announced that Aitken would be a full-time IMSA driver in 2024 for Action Express Racing alongside Pipo Derani.

[43] Aitken joined Envision Racing in Formula E to partake in the rookie test at the Tempelhof Airport Street Circuit during late April 2023.