Juan was born in
Carrasco, Montevideo to an Argentine father and a Uruguayan mother, and came through the
youth system at
Miramar Misiones. He was promoted to the first team in the
2010–11 Uruguayan Primera División season, making two substitute appearances in the campaign. His debut came in a 5–2 loss against
Danubio F.C. on 20 February 2011, when he was a 64th-minute substitute. His second match of the 2010–11 season, again as a substitute, was a 0–0 draw with
Bella Vista on 8 May 2011. This was his first appearance in home stadium
Parque Luis Méndez Piana. He scored his first goal with
Miramar Misiones on 22 December 2011 against
Deportivo Maldonado in the 2011–12 season of the
Uruguayan Segunda División.
NorthEast United FC On 27 August 2018, Mascia moved abroad and joined
Indian Super League franchise
NorthEast United FC. On 30 October 2018, Mascia made his debut for the club on away win against
Delhi Dynamos. On 23 November 2018, he scored his first goal for the club against
Kerala Blasters in the final moments of the game after coming as a substitute and hence, earned the nickname 'Super Sub Mascia'. He is also called 'Boot Jolokia' after
Bhoot Jolokia for his goal against Kerala Blasters by the fans. Along with teammate
Bartholomew Ogbeche and compatriot
Federico Gallego, he formed one of the best attacking combinations for the club.
Chacarita Juniors In June 2019 his transfer was announced to
Chacarita Juniors of
Uruguay. == International career ==