Hartley began playing rugby union at the age of five, following in the footsteps of his father Gary – a former trialist with the England national team – when he joined Hammersmith & Fulham RFC.
[3] On 17 February 2022, two days before his 20th birthday, it was announced that Hartley had signed a long-term deal with Wasps, graduating into the senior squad for the 2022–23 Premiership season.
One month later, he signed an academy contract with Saracens in November 2022, and his first appearance for them soon followed, during a pool stage fixture against Leicester Tigers in the 2022–23 Premiership Rugby Cup.
He subsequently made his European Rugby Champions Cup debut in December 2023, as a substitute against the Bulls, then earned his first start in the competition a week later, scoring two tries in a win over Connacht.
[9] In total, Hartley made 20 first-team appearances for Saracens across all competitions, before suffering a serious season-ending knee injury at the end of a Premiership match against Gloucester in April 2024.