*Club domestic league appearances and goals, correct as of 16:20, 7 June 2024 (UTC) ‡ National team caps and goals, correct as of 20:08, 4 March 2024 (UTC)
Having progressed through the academy of local club Linfield, Allen made his senior debut for the club on 27 April 2019 in a 1–1 draw with Coleraine.[3] Upon making his debut, aged 15 years, five months and five days, he became the club's youngest debutant and set teammate Daniel Reynolds up for Linfield's goal.[4]
On 27 July 2020, the Yorkshire Evening Post revealed that Allen had agreed a deal with newly-promoted Premier League side Leeds United.[6] The move was officially announced on 11 August, with Allen signing a three-year deal.[7] Allen made his competitive debut for Leeds by playing for Leeds United Under 21's playing in the EFL Trophy defeat against Accrington Stanley on 8 September 2020, Allen was given the squad number 70 for the 2020–21 Leeds United season.[8] He signed his first professional contract with Leeds United in December 2020, agreeing a deal lasting until summer 2023.[9] In August 2022 his contract was extended to the end of the 2023–24 season.[10]
Allen was released by Leeds in June 2024 at the conclusion of his contract.[11]
Return to Linfield
On 9 August 2024, Allen returned to NIFL Premiership side Linfield on a two-year contract.[12]