[4][5] Born in San Sebastián, Gipuzkoa, Prieto was a graduate of the youth system at his hometown club Real Sociedad, having joined from school aged 16 and spent time on loan with local amateurs CD Hernani.

[6] His La Liga bow took place 18 days later, coming off the bench in the 1–0 home victory over Navarrese neighbours CA Osasuna;[7] he would go on to collect a further eight substitute appearances – seven of them late in the game – during that season.

On 25 August 2017, Prieto played his 500th game for the Real Sociedad first team (only the fifth player to reach that milestone after Juan Antonio Larrañaga, Alberto Górriz, Jesús María Zamora and Luis Arconada) a few days prior to his 34th birthday, and marked the occasion with a goal in a 3–0 victory against Villarreal at the Anoeta Stadium.

He also received an ovation from supporters in the tenth minute, his shirt number, and a few weeks later he broke the club record for most league appearances (including matches he played in the lower division, unlike the players he overtook).

To honour his contributions, Real Sociedad arranged for the club crest to be replaced with his portrait on the players' shirts for the final match of the campaign against CD Leganés (which would also be the swan song for long-serving defender Carlos Martínez).