He made his debut for Polonia Warbud Warszawa in 2003–04, seeing limited action off the bench for the team.

However, the following year, he saw significantly more action, averaging 11.6 points and 4.8 assists per game in leading the team to a third-place finish in the league.

[2] After a successful season in Anwil, he moved to Italy and signed a two-year contract with JuveCaserta, where he was just a backup point guard and didn’t see much playing time.

In the 2011–12 season Koszarek was voted the MVP of the league and led Trefl to finals, where they lost to Asseco Prokom.

He was also a member of the Polish national team at Basketball World Cup 2019 in China,[5] where he averaged 2.5 points, 0.8 rebounds and 2.5 assists in 14 minutes per game.