Franco Mastantuono was born on 14 August 2007 in Azul, Buenos Aires Province, Argentina, of Italian-Argentine heritage. He came through River Plate's academy and made his senior debut in January 2024, becoming River's third-youngest debutant ever. The following month he scored his first senior goal in a Copa Argentina match, becoming the club's youngest-ever goalscorer and breaking a record that had stood since Javier Saviola.
Real Madrid had tracked him for nearly a year and announced his signing on 13 June 2025, triggering a release clause worth €45 million; total package costs of around €63.2 million made it the largest fee ever paid for a player from Argentine football. He officially joined the club on his 18th birthday, 14 August 2025, on a six-year contract through June 2031, taking the number 30 shirt. He made his Real Madrid debut on 19 August 2025, and on 16 September 2025 became the club's youngest-ever Champions League starter, surpassing Endrick's record by 40 days.
A left-footed attacking midfielder or right winger who can also operate as a classic enganche, Mastantuono is known for his striking technique, free-kick prowess, and one-on-one ability. He has named Phil Foden, Neymar, and Julián Álvarez as influences. He made his senior debut for Argentina on 5 June 2025 against Chile aged 17 years and 296 days — his country's youngest ever in an official match — and is widely projected as one of the leading South American talents of his generation.