Juan was born in Carrasco, Montevideo to an Argentine father and a Uruguayan mother,[1] and came through the youth system at Miramar Misiones.

[5] On 27 August 2018, Mascia moved abroad and joined Indian Super League franchise NorthEast United FC.

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'.

Juan entered the Uruguay under-15s in 2009 and his first official international tournament was the 2009 South American Under-15 Football Championship in which he played six matches and scored three goals.

[8] He scored another double in a 3–2 win against the Colombia under-17s in a final group stage game on 6 April 2011, helping Uruguay finish the competition as runners up to champions Brazil.

[13][14] In the same month, Uruguay newspaper El Observador reported that Forlán was, in fact, a keen admirer of the player and had arranged for him to attend a training session with Atlético Madrid.