Lountzis was invited to the Next Generation Practices at the 2014 Adidas Eurocamp, in Treviso, Italy,[2] and, earlier that year, to the Jordan Brand Classic International Tour.

Lountzis played with Kronos Agiou Dimitriou in Greece's amateur level fourth division, in the 2013–14 season.

On 23 June 2014, Lountzis signed a five-year deal with Panathinaikos of Greece's top-tier level Greek Basket League.

[3][4] Lountzis made his professional debut at age 16, with Panathinaikos, on January 12, 2015, in a Greek Basket League victory over Panelefsiniakos.

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.