The St. Louis Cardinals signed Koperniak as an undrafted free agent on June 16, 2020.[7] He was assigned to the Palm Beach Cardinals of Low-A Southeast at the start of the 2021 season.[8] He slashed .322 /.443 /.470 in 58 games with Palm Beach before being promoted to Peoria Chiefs of High-A Central at the beginning of August. Koperniak had six hits in 14 at-bats over four games for the Chiefs and was promoted a second time to the Double-ASpringfield Cardinals.[9] Over 95 games total between the three clubs, he hit .306 with seven home runs, 41 RBI, and 25 doubles.[10]
He returned to Springfield for the 2022 season and also played in two games for the Triple-AMemphis Redbirds.[11] Over 108 games between the two teams, Koperniak slashed .291/.364/.443 with 14 home runs, 59 RBI, and 12 stolen bases.[12]
Koperniak split time between Springfield and Memphis again in 2023, slashing .287/.362/.427 with 18 home runs, 85 RBI, and 11 stolen bases over 129 games.[13]
Koperniak played in 122 games for the Memphis Redbirds in 2024, slashing .309/.370/.512 with 20 home runs and 73 RBI. Following the season, the Cardinals added Koperniak to their 40-man roster to protect him from the Rule 5 draft.[14]