On 16 October 2010, Mallo scored his first career goal in a 2–2 draw at Villarreal CF B.[6] He started in all 34 matches he appeared in the 2011–12 campaign, as the side finished second and returned to La Liga after five years; he made his debut in the competition on 18 August 2012, in a 0–1 home defeat to Málaga CF.[7]
On 5 May 2018, Mallo made his 300th competitive appearance in another local derby, putting him sixth in their all-time veterans.[12] That August, he turned down a move to Premier League newcomers Fulham,[13] instead signing a new five-year contract with a buyout clause of €50 million.[14] On 24 November 2019 he moved joint-fourth on the list with a 350th appearance at Villarreal CF, and at 28 he became the youngest player to reach that figure for the club.[15]
By the time that he turned 30 in June 2021, Mallo had beaten Aleksandr Mostovoi's record of 235 top-tier games for Celta.[16] On 15 August that year, his added-time foul on Luis Suárez in a 2–1 home loss to Atlético Madrid caused a brawl which resulted in dismissals for himself, opponent Mario Hermoso and Atléti fitness coach Óscar Ortega.[17] He returned from suspension 13 days later in a 1–0 defeat at the same venue to Athletic Bilbao, his 400th match.[18]
Mallo left on 8 June 2023, after his contract expired; at the time of his departure, he was the club player with the most appearances in the main division at 309.[19]
In September 2011, Mallo made his first under-21 appearances in home and away wins over Georgia in European qualification. He then lost his place to Montoya, missed the 2012 Olympics and was edged from even his substitute place under Luis Milla for a year until being recalled against Italy in November 2012, his final international.[27]
On 30 January 2024, Mallo was ordered to stand trial in Barcelona's Criminal Court number 19 for allegations that he inappropriately touched the breasts of a woman who wore a parakeet costume while working as an Espanyol mascot. The incident, which was caught on film, took place in a Celta match at the Cornellà-El Prat Stadium on 24 April 2019; due to the penal code which was in force at the time, he could be charged with sexual abuse rather than sexual assault, with the trial being scheduled to start on 11 July 2024.[29]
The accuser, who remained anonymous and used the alias "Ms. Ana," previously testified about the incident while appearing before the Court of First Instance and Instruction Number 2 of Cornellà de Llobregat on 27 June 2019. On 25 September, the court would give the case a provisional dismissal. However, on 14 November, it would deny Mallo a "free dismissal" which would have closed the case. On 31 May 2021, the Ninth Section of the Provincial Court of Barcelona revoked the court's dismissal; he maintained his innocence, claiming that he did not know the gender of the person in the parakeet costume at the time of the incident and that he greeted Espanyol's mascots and players normally.[29]
^Bernardo, Julio (10 May 2018). "Tras los pasos del Gran Capitán" [In the footsteps of the Great Captain]. Faro de Vigo (in Spanish). Retrieved 27 April 2020.