[4] In 2001, aged 12, he joined local Real Madrid's youth academy from neighbouring amateurs CD Coslada.

[6][7][8][9][10] As first-team manager José Mourinho decided to rest Iker Casillas, Mejías was selected for a La Liga home match against Getafe CF on 10 May 2011.

[19] On 28 October of the following year, he saved from Wayne Rooney and Ashley Young as his team won in a penalty shoot-out away to Manchester United in the fourth round of the League Cup, also keeping a clean sheet for 120 minutes.

[28] On 4 January 2023, Mejías returned to Spain six years after leaving, agreeing to a contract at Primera Federación club AD Ceuta FC.

[29][30] On 1 February 2024, having become a backup to new signing Pedro López,[31] he returned to the professional leagues on a five-month deal at second-tier FC Cartagena.