Mac Allister scored his first Brighton goal on 17 September 2020, with in a header in a 4–0 home victory over Portsmouth in the EFL Cup. He scored again six days later, this goal also coming in the EFL Cup where Brighton claimed a 2–0 away victory over Preston. On 18 October, Mac Allister scored his first Premier League goal scoring a 90th minute leveller in a 1–1 away draw against bitter rivals Crystal Palace.
