Franco Mastantuono was born on August 14, 2007 in Azul, a small town in the province of Buenos Aires. He was always a precocious talent – such that he was named as one of the top ten Argentine tennis players of his age. Despite having a father who was also a football coach, he only decided to focus solely on football at the age of 11.

He started out at Club Atletico River Plate Azul – where his father coached – then joined Cemento Armado FC for two years before making the switch to the River Plate academy. There he quickly established himself as one of the country's brightest young prospects, prompting River Plate coach Martín Demichelis to hand him his senior debut on January 28, 2024 at the age of 16 years, five months and 14 days, in a 1-1 draw with Argentinos Juniors. Eleven days later, he became the youngest goalscorer in River Plate history with his strike in a Copa Argentina game against Excursionistas, Before long, he was a first-team regular. 

At the age of 17 he became the youngest debutant for the senior Argentina team and he went on to become the youngest player to wear the Argentina No.10 jersey, against Ecuador.

After 64 appearances, ten goals and seven assists for River Plate (with seven goals and four assists in 22 matches in his final season for the club), he earned himself a move to Real Madrid. 

Mastantuono made an encouraging start (11 games, seven as a starter) in the Spanish capital, but the fierce competition for places reduced his number of starts. He nonetheless ended his first season at Real with 35 appearances and three goals (one in La Liga, one the Copa del Rey and one in the Champions League).