[3] Prieto made occasional appearances in La Liga for Málaga and in the Segunda División for Real Madrid Castilla, and played regularly at Segunda División B level over three seasons with Espanyol B, before spending a season in English football as backup goalkeeper at EFL Championship club Birmingham City.

Despite appearing mainly with the Juvenil A squad, Prieto made his professional debut on 13 April 2013, replacing the injured Tomás Mejías in a 3–1 away success against Recreativo de Huelva in the Segunda División championship.

[10] Initially a backup to Diego López, he dropped to third choice after the arrival of Oier Olazábal, and terminated his contract with the club on 28 August 2020.

[12] He made his debut in the opening fixture of the season, a 1–0 defeat at home to fourth-tier Cambridge United in the EFL Cup; the goal came when Prieto was outjumped at a set-piece.

[13] He was a regular on the bench behind first-choice Neil Etheridge for most of the season,[14] but his only other first-team appearance was in the 3–0 FA Cup defeat away to Manchester City.