Career
Panathinaikos
Mavrias joined Panathinaikos" youth academy in 2007, aged 13, and was promoted to the first-team squad in 2010, after signing a professional contract in the previous year. On 20 October 2010 he made his first-team – and Union of European Football Associations Champions League – debut, playing the last 12 minutes of a 0–0 home draw against Rubin Kazan, thus becoming the youngest Greek ever to appear in the competition, and the second youngest overall (only behind Celestine Babayaro, being surpassed later by Alen Halilović and Youri Tielemans). Four days later he made his league debut, again as a substitute in a 0–1 loss at AEK. Sunderland
On 22 August 2013, Mavrias joined English Premier League side Sunderland on a three-year contract, for an undisclosed fee, rumoured to be £2-3 million.
However, he was left out of the squad to play Southampton due to lack of match fitness.
Mavrias made his debut five days later, coming on as a second-half substitute in a 4–2 home success over Milton Keynes Dons, for the campaign"s Football League Cup. On 2 February 2015, it was confirmed Mavrias had joined his former club Panathinaikos on loan until the end of the 2014-2015 season.
Mavrias returned to Sunderland where he has been training and playing for the Black Cats Under-21s – and is understood to have impressed the club’s coaching staff with his attitude and contribution. But Mavrias has not come close to being included in the first-team squad, however is not bitter over seeing his career stall on Wearside.
“I took the decision to leave Panathinaikos and go to Sunderland, and I think anyone in my position would take this decision,” he told the Greek press
Fortuna Düsseldorf
He was loaned to Fortuna Düsseldorf on 27 January 2016. Mavrias - who has 18 months remaining on his Sunderland contract - now has a chance to play regular first-team football after joining Dusseldorf until the end of the campaign, with a view to a permanent switch next summer. On 6 February 2016, he made his debut with a club, in a 0-1 home loss against Heidenheim.