Alexis Mac Allister was born in Santa Rosa, La Pampa, Argentina, to a family with a strong football background. His older brothers Francis and Kevin are also footballers, and his father Carlos Mac Allister and uncle Patricio Mac Allister are retired footballers.

Mac Allister made his professional debut on February 29, 2016, for Argentinos Juniors in a match against Independiente. This marked the beginning of his senior career, where he quickly impressed with his skills and became an important player for the club.

In January 2019, Mac Allister signed a contract with Brighton & Hove Albion but was immediately loaned back to Argentinos Juniors for the remainder of the 2018-2019 season. This move marked his entry into English football, although he initially continued to play in Argentina.

For the 2019-2020 season, Mac Allister was loaned to Boca Juniors, where he played a crucial role in the team's campaign in the Copa Libertadores and the Argentine Superliga. This period was significant for his development and exposure at a higher level.

After his successful loan spell at Boca Juniors, Mac Allister returned to Brighton & Hove Albion for the 2020-2021 season. He managed to secure more playing time and showcased his abilities in the Premier League, establishing himself as a key player for the team.

Mac Allister made his senior international debut for Argentina in 2021, fulfilling a childhood dream. He has since become a regular member of the national team, contributing to several major tournaments.

Mac Allister was part of the Argentina squad that won the 2022 FIFA World Cup in Qatar. His performances in the midfield were vital to the team's success, providing stability and creativity throughout the tournament.

On June 8, 2023, Mac Allister signed a five-year contract with Liverpool, marking a significant step in his career. He made his debut for the club on August 13, 2023, in a Premier League match against Chelsea.

Mac Allister's first season at Liverpool was marked by key contributions across competitions. He scored his first goal for the club on December 3, 2023, in a 4-3 win over Fulham, which was later voted the Premier League Goal of the Month for December.

As of January 2025, Mac Allister has been in excellent form for Liverpool, making 21 appearances, scoring 1 goal, and contributing 3 assists in the Premier League. His consistency and passing accuracy have been crucial to Liverpool's success.