Lisandro Martínez was born on 18 January 1998 in Gualeguay, Entre Ríos Province, Argentina. He came through youth football at local clubs before joining the academy of Newell's Old Boys, where he made his professional debut in the final match of the 2016–17 season. A loan move to Defensa y Justicia followed, and after impressing there he earned a permanent transfer to Dutch giants Ajax in July 2019 for a fee of around €7 million.
At Ajax, Martínez made 120 appearances over three seasons, winning two Eredivisie titles, the KNVB Cup, and the Johan Cruyff Shield. He was voted Ajax Player of the Year for the 2021–22 season, capping a strong stint in Amsterdam.
In July 2022, Manchester United signed him for a reported fee of up to £57.5 million including add-ons. He quickly became a key figure in central defence, earning the club's Player of the Month award in his first August at Old Trafford. He picked up his first trophy with United in the 2023 EFL Cup final and added an FA Cup winner's medal in 2024. Injuries have disrupted his time in Manchester, but he has shown his importance when fit.
An aggressive, left-footed centre-back with composure on the ball and a strong range of passing, Martínez has earned comparisons to former United defender Nemanja Vidić. He made his senior Argentina debut in March 2019 against Venezuela and has since been part of squads that won the 2021 Copa América, the 2022 FIFA World Cup, and the 2024 Copa América.