The 19-year-old winger has finally completed his protracted £2.5m move from Panathinaikos, signing a four-year deal with the Wearsiders.
Greece international Mavrias - who prefers to be known as Harry - told the Evening Chronicle: "It was the right time for me to make the move.
"I am 19 years old and last year I played a full championship [season] - a lot of games.
"But this is the best time for me to make the next step with Sunderland."
Mavrias is the 11th new arrival on Wearside this summer, as part of boss Paolo Di Canio's squad revamp.
"This is an exciting time to join Sunderland," said Mavrias. "I think the changes will bring good results.
"The Premier League is the best [league] in the world and the difference between [it] and Greece is very big, because the level in England is very high."
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