Mantzaris began his professional career in the Greek 2nd Division with Peristeri, the team of his hometown. With Peristeri, he won the Greek 2nd Division championship in 2009.
In January 2013, Mantzaris suffered a season-ending knee injury, after rupturing the anterior cruciate ligament in his right knee during a Greek Basket League derby against PAOK Thessaloniki at PAOK Sports Arena.[2] Despite suffering the knee injury, he remained close to his teammates both in and out of the locker room in order to support them, as they won the EuroLeague 2012–13 season championship. Mantzaris returned to action seven months later and in early October of the same year won the FIBA Intercontinental Cup against Pinheiros at Ginásio José Corrêa, Brazil. In the two finals, he averaged 9 points, 4.5 rebounds and 3 assists, helping Olympiacos to celebrate the title. On 29 July 2014, Mantzaris extended his contract with Olympiacos through 2017.[3]
On June 16, 2017, Mantzaris signed a three-year contract extension with Olympiacos.[4]
UNICS Kazan
On July 20, 2019, Mantzaris was released from Olympiacos, after eight seasons, and signed with UNICS Kazan of the VTB United League and the EuroCup, playing under fellow Greek coach Dimitrios Priftis. He left in December, returning to Greece.
Promitheas Patras
On December 30, 2019, Mantzaris returned to Greece and signed with EuroCup side Promitheas Patras. He averaged 10.5 points, 2.2 rebounds, 5.3 assists and 1.8 steals per game. On October 8, 2020, Mantzaris re-signed with the team.[5]
Return to Peristeri
On December 17, 2020, Mantzaris officially returned to Peristeri after nine years.
Mantzaris is a resident of Peristeri, which is a west suburb of Athens, and the biggest municipality of the Attica Region. His brother, Antonis Mantzaris, is also a professional basketball player. They were teammates from 2007, until the summer of 2011, with Peristeri Basketball Club. In 2009, they won the Greek 2nd Division championship with the club. He owns a café in the area, as well.
