Mac Allister, like his two brothers, started his career with Argentinos Juniors.[1] His first appearance for the Argentinos first-team came on 27 February 2016, in a 4–1 defeat to Estudiantes.[2][3] Three more appearances followed in his debut season of 2016. The club was relegated in that campaign, Mac Allister subsequently made sixteen appearances in Primera B Nacional as Argentinos won the title.[2] He scored his first senior goal in March 2018 against Gimnasia y Esgrima.[2] Mac Allister joined Boca Juniors on loan in January 2019.[2][4]
Union SG
On 12 July 2023, Belgian club Union SG announced the signing of Mac Allister on a three-year contract, with an option of a further year.[5]