[1] Mastrogiannopoulos started his professional career with the Greek 1st division club Apollon Patras in 2013.
On July 3, 2018, Mastrogiannopoulos joined Lavrio of the Greek Basket League on a two-year contract.
In 27 games, he averaged career-highs of 15.3 points, 4.2 rebounds, 3.1 assists and 1.7 steals per contest.
On August 13, 2022, Mastrogiannopoulos made his return to Apollon Patras after four years.
In 21 league games, he averaged 5.4 points, 1.8 rebounds and 1 assist in 16 minutes per contest.