Alexis Mac Allister was born on 24 December 1998 in Santa Rosa, La Pampa, Argentina, into a footballing family — his father Carlos and uncle Patricio both played professionally, and his brothers Francis and Kevin are active professionals. Mac Allister came through the youth ranks at Argentinos Juniors, making his senior debut in October 2016 and helping the club win promotion from the Argentine Second Division in his first season. Brighton & Hove Albion signed him in January 2019, though he spent time on loan at Argentinos Juniors and Boca Juniors before establishing himself at the English club.
His breakthrough at Brighton came during the 2020–21 season, and he went on to become the club's talisman, finishing as their top scorer with 10 league goals in 2022–23 as Brighton qualified for European football for the first time in their history. He was voted Brighton's Player of the Season by BBC Sport users that year.
A composed, versatile midfielder with strong passing range and an eye for goal, Mac Allister signed for Liverpool in June 2023 for a reported initial fee of £35 million. At Liverpool he has developed into a key deep-lying midfielder under manager Arne Slot, contributing to the club's 2023–24 EFL Cup and 2024–25 Premier League title wins.
On the international stage, Mac Allister made his senior debut for Argentina in September 2019 against Chile and has since been part of squads that won the 2022 Finalissima, the 2022 FIFA World Cup, and the 2024 Copa América.