Hartley made his first-team debut for Wasps in 2021, at the age of 19, playing against Newcastle Falcons in the pool stages of the 2021–22 Premiership Rugby Cup.[2] He then spent the majority of the year on loan with Ampthill in the RFU Championship, making four appearances and scoring two tries for his parent club that year.[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.[4]
On 17 October 2022, Wasps were placed into administration due to financial insolvency and immediately ceased trading, with all players and staff at the club made redundant.[5] This meant that Hartley never featured in a Premiership match for the club. 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.[6]
Hartley made his Premiership debut for Saracens on 25 February 2023, starting at inside centre in a home game against Newcastle Falcons.[7] He earned plaudits for his performance and was voted as player of the match.[8]
Ahead of the 2023–24 season, Hartley was promoted into the Saracens senior squad, while also dual-registered with Championship club Ampthill. 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.[10] Despite sustaining a ruptured anterior cruciate ligament, a torn meniscus, a ruptured medial collateral ligament and a torn posterior cruciate ligament, he returned to action for Saracens just seven months later, during the 2024–25 Premiership Rugby Cup.[11]