He made his debut on 12 September in the first round of the Greek Cup, and his first league appearance on 19 October 2014 at home to Apollon Smyrnis: with his team losing 1–0, Mantalos took the free kick from which Alexandre D'Acol equalised and then converted a penalty to complete a 2–1 win.

[citation needed] Mantalos himself missed the last few weeks of the season, having ruptured his anterior cruciate ligament while playing in a cup derby against Olympiacos in March 2015.

After six months out, Mantalos returned to competitive action and continued as a regular in the first team, helping AEK finish third in the 2015–16 season and qualify for the Europa League.

[8] According to an end-of-season profile, Mantalos "endured a difficult start to the campaign",[9] which was interrupted by the need for surgery on a nose broken in training,[10] "but seemed rejuvenated once Manolo Jiménez took over the team" in January 2017.

[19] Mantalos converted a penalty in a 1–1 draw in the Champions League play-off round second leg against Vidi that, despite having two men sent off, was enough to progress to the group stage on aggregate.

[22] His fourth league goal, which opened the scoring in a 2–0 win away to Panionios in January 2019, was controversial as he appeared to handle the ball when controlling the final pass.

[28] On 18 December 2019, Mantalos opened the scoring from the penalty spot after six minutes to put Massimo Carrera’s AEK in front in a 2–1 home win game against Asteras Tripolis.

[30] On 4 March 2020, Marko Livaja picked out the onrushing Mantalos on the left, and the latter calmly stroked the ball into the far corner beyond Julián Cuesta, opening the score in a slender 2–1 advantage against Aris in Athens, with the second leg of the Greek Cup semi-final to be played at the Kleanthis Vikelidis Stadium next month.

[31] At the end of the regular season, Mantalos has showed a consistency throughout the entire campaign and as a result clubs from Turkey, including Super Lig leaders İstanbul Başakşehir, are reportedly eager to sign the 28-year-old in the upcoming transfer window.

[37] On 9 May 2021, he scored a vital goal sealing a 1–0 away victory against rivals Panathinaikos in his club effort to gain the last ticket for 2021–22 UEFA Europa Conference League.