Mantzaris became a member of the senior men's Greek national basketball team in 2011. He was a member of Greece's senior national teams that played at the 2012 FIBA World Olympic Qualifying Tournament, the 2014 FIBA World Cup, and the EuroBasket 2015. He also played at the 2016 Turin FIBA World Olympic Qualifying Tournament, and at the EuroBasket 2017.