Hartley was born in Palmerston North and began karting at age six. After winning the 2003 New Zealand Formula Ford Festival, he moved to Europe and competed in the Eurocup Formula Renault 2.0, winning the championship in his second season. He progressed through British Formula Three, finishing third in 2008 with Carlin Motorsport, and into Formula Renault 3.5 with Tech 1 Racing in 2010, where he was paired with fellow Red Bull Junior driver Daniel Ricciardo. Mid-season, Hartley was dropped from the Red Bull Junior Team and his path to Formula One was deferred.
With no competitive single-seater drive available, Hartley joined Murphy Prototypes in the European Le Mans Series in 2012, contesting the LMP2 class. He immediately showed endurance potential, finishing third in the LMP2 class at the 6 Hours of Spa-Francorchamps on his effective WEC debut.
Ahead of the 2014 season, Porsche signed Hartley as a factory driver for its LMP1 Porsche 919 Hybrid programme. His first season was difficult, but in 2015 he partnered Mark Webber and Timo Bernhard to win four consecutive races and clinch both the drivers' and manufacturers' WEC titles. The trio defended their title in 2016 and Hartley was again a title contender, recording multiple race wins, before finishing fourth in the championship. In 2017, now alongside Bernhard and compatriot Earl Bamber, Hartley's No. 2 Porsche won the 24 Hours of Le Mans after recovering from an 18-lap garage stop caused by a hybrid failure, as rival LMP1 cars struck trouble. The crew then secured the 2017 WEC drivers' championship, completing a remarkable hat-trick for Hartley in his Porsche career.
Hartley made his Formula One debut at the 2017 United States Grand Prix, replacing Pierre Gasly at Toro Rosso. He was retained for the remainder of 2017 and confirmed as a full-time driver for 2018. In a difficult season with an uncompetitive package, Hartley scored points at the Azerbaijan, German, and United States Grands Prix, with a career-best ninth place at the latter. He was not retained for 2019 and was replaced by Alexander Albon.
Hartley returned to the WEC in 2019, initially racing for SMP Racing at Sebring before being announced as the replacement for Fernando Alonso in Toyota's No. 8 GR Super Sport Concept alongside Buemi and Kazuki Nakajima. The 2019–20 season brought the crew's first Le Mans victory together in 2020, when the sister No. 7 Toyota retired with an exhaust manifold failure, leaving the No. 8 to win unchallenged.
In 2021, Toyota entered the new Hypercar class. Hartley and his crew won at Spa and Portimão but were denied the Le Mans victory by a lap-1 collision, ultimately finishing second. The No. 8 finished as runners-up in the 2021 championship. The 2022 campaign was more rewarding: Hartley took pole at Le Mans and drove to an outright victory there — his first Le Mans win with Toyota — and the crew added another win at Fuji before clinching the drivers' championship at the final round in Bahrain. In 2023, Hartley and his teammates won the drivers' championship again, with a string of top-two finishes across the season including another Le Mans runner-up result as Ferrari took the historic victory.
The 2024 season proved more competitive against a resurgent Hypercar field. The No. 8 struggled for pace, finishing fifth at Le Mans, though Hartley guided the crew to victory at Bahrain in a dramatic season finale, clinching the manufacturers' title for Toyota. In 2026, Hartley and his teammates capitalised on strategy at Imola to score Toyota's 50th WEC victory in the marque's 100th WEC start.
Hartley's career trajectory — from Red Bull junior dropped before reaching Formula One, to a dominant endurance career spanning Porsche's LMP1 era and Toyota's Hypercar years — is one of the sport's more compelling reinventions. His four WEC titles, three Le Mans victories, and record of consistency across multiple machinery eras place him among the pre-eminent endurance specialists of the 2010s and early 2020s.