Hartley began karting aged six and progressed through New Zealand Formula Ford, Formula Toyota, and Eurocup Formula Renault 2.0, winning the Eurocup title in his second season. He competed in British Formula Three in 2008 with Carlin, winning five races and finishing third in the championship. He spent time in the Formula Renault 3.5 Series alongside Daniel Ricciardo as a fellow Red Bull junior driver, but Red Bull dropped him from their programme after the 2010 season. Brief stints in GP2 followed before Hartley shifted his focus to sportscar racing.
Porsche signed Hartley as a factory driver for the 2014 World Endurance Championship season. In 2015 he, alongside Mark Webber and Timo Bernhard, won the WEC drivers' title for Porsche. He defended the title in 2017 alongside Bernhard and Earl Bamber, a season in which the trio also won the 24 Hours of Le Mans after overcoming an 18-lap mechanical delay. Porsche withdrew from the WEC LMP1 class at the end of 2017.
Hartley joined Toyota for the 2019–20 WEC season, replacing Fernando Alonso in the No. 8 car alongside Sébastien Buemi and Kazuki Nakajima. The crew won the 2020 Le Mans 24 Hours after the sister Toyota retired from contention. In 2022, Hartley, Buemi, and new partner Ryo Hirakawa won the drivers' championship and added a third Le Mans victory for the crew. The season also saw them win the 2022 WEC drivers' title. The partnership won a fourth WEC title in 2023. By the time of the 2024 and 2025 seasons, results for the No. 8 crew became less consistent, though Hartley and his teammates claimed a victory at São Paulo in 2024 and helped Toyota win the manufacturers' championship at Bahrain. In 2026, the No. 8 crew won the season opener in Imola, marking Toyota's 50th WEC victory in its 100th start.
After Porsche's LMP1 withdrawal, Hartley was given a Formula One debut at the 2017 United States Grand Prix with Toro Rosso, replacing Pierre Gasly for that event. He was confirmed as a full-time driver for 2018 alongside Gasly. His best finish in Formula One was ninth place at the 2018 United States Grand Prix. He scored additional points at the Azerbaijan and German Grands Prix that year. He was replaced by Alexander Albon for 2019. Following his F1 spell, Hartley briefly drove in Formula E with GEOX Dragon Racing before departing mid-2020 and returning to the WEC.
Hartley is one of the most decorated endurance racing drivers of his generation, tied for the most WEC titles in history at four. He holds three Le Mans victories and is known for consistency and racecraft across LMP1, Hypercar, and multi-class environments. He lives in Monaco and continues to compete for Toyota in the WEC.