Mastantuono's journey began in the River Plate youth academy in 2019, where he developed his skills until 2024. During this time, he became a part of the first team in the 2024-2025 season. In February 2024, he made history by becoming the youngest goal scorer in River Plate's history, and he also won an Argentine Super Cup with the team.

At just 17 years old, Mastantuono is also the youngest player to feature in an official match for Argentina's national team. His impressive achievements have caught the attention of Real Madrid, who have signed him for a reported €45 million fee.

With this new signing, Real Madrid's manager Xabi Alonso hopes to find a suitable replacement for the departing Luka Modric. Mastantuono is expected to play a key role alongside Jude Bellingham in dictating the tempo of games next season. The team will be looking to make a strong comeback after missing out on several titles, including the Copa del Ray, the Champions League, and La Liga.