Lefteris Mantzoukas was born in July 8, 2003, he is a 2.07 m (6'9 1⁄2") tall small forward-power forward.
On 14 February 2017, at the age of 14, Mantzoukas began his pro career with Promitheas Patras, in the Greek Basket League, during the 2017–18 season. He made his Greek League debut in a game versus Panionios, in which he scored 2 points, in 1 minute of playing time. Mantzoukas became the second youngest player to ever debut in the top-tier level Greek League, since it became fully professional in 1992. Georgios Papagiannis is the only other player to have previously debuted in the same competition, at a younger age.
Mantzoukas has been a member of the junior national teams of Greece. He played at the 2018 FIBA Under-16 European Championship, where he averaged 9.3 points, 4.8 rebounds, and 1.4 assists per game. He also played at the 2019 FIBA Under-16 European Championship, where he averaged 15.0 points, 9.1 rebounds, and 2.7 assists per game.