On 23 June 2014, Lountzis signed a five-year deal with
Panathinaikos of Greece's
top-tier level Greek Basket League. Lountzis made his
professional debut at age 16, with Panathinaikos, on January 12, 2015, in a Greek Basket League victory over
Panelefsiniakos. He became the youngest player ever to play in the
Greek Cup Final, when he played with Panathinaikos in their victory against
Apollon Patras, in the
2015 Greek Cup Final, on April 5, 2015. He made his debut in the
European-wide top-tier level EuroLeague, at the age of 16, on April 14, 2015, in a playoff game against
CSKA Moscow. On August 21, 2017, Lountzis was
loaned to
Trikala for the
2017–18 season. On July 27, 2018, Lountzis was
loaned once more, this time to
Lavrio for the
2018–19 season. On July 2, 2019, Lountzis was released from his contract with
Panathinaikos after five years. On August 5, 2019, Lountzis agreed to a new two-year (1+1) contract with
Lavrio. He averaged 5.9 points, 2.6 rebounds and 1.25 assists per game. On July 24, 2020, Lountzis moved to
Promitheas. On July 6, 2021, Lountzis signed a three-year deal with
Olympiacos, where he spent three seasons mostly as an auxiliary player. On July 29, 2024, Lountzis parted ways with Olympiacos and returned to Promitheas. On June 25, 2025, Lountzis joined
Panionios of the
Greek Basketball League and the
EuroCup. In November 2025, he suffered an
anterior cruciate ligament tear and would need surgery. He was going to be sidelined for at least six months. He previously suffered a torn cruciate ligament in 2019. ==National team career==