She made her debut as an actress in 1997, in the miniseries Un prete tra noi, followed by Amico mio 2 (1998), directed by Paolo Poeti, Lui e lei 2 (1999), Il veterinario, by José María Sánchez, and Il grande Torino with Remo Girone and Michele Placido, directed by Claudio Bonivento, in which she pictures the character of Rosa Di Girolamo, both filmed in 2005, and an episode of Don Matteo 5 (2006).

Mastronardi gained more fame, mainly in Italy, when she became a regular cast member of the 2006 Italian version of the TV series Los Serrano, I Cesaroni, in which she plays the part of the young shy and romantic girl Eva Cudicini.

She portrayed the character of Margherita, while the role of her fiancé Mamo was played by Eros Galbiati, who became popular in Italy in 2006 thanks to the movie Notte prima degli esami.

Subsequently, Alessandra obtained an important role in TV film Non smettere di sognare, in which she played the part of a young dancer, Stella.

Mastronardi's first video game role came in 2024's Indiana Jones and the Great Circle where she played Italian investigative journalist Ginetta 'Gina' Lombardi.