On May 24, 1949 — Striking out the last 6 St. Paul batters‚ Mickey McDermott of Louisville (American Association) fans a total of 20 for a new league record. McDermott wins‚ 3 – 1‚ striking out the side in the 3rd‚ 5th‚ 6th‚ 8th‚ and 9th innings.
Source:
Baseball Reference May 24