St. Louis selected Leahy's contract from Triple-A Memphis on Thursday.
Leahy is up to fill a long-relief role for the Cardinals, who badly need fresh arms in the bullpen. The 26-year-old right-hander was showing an underwhelming 4.06 ERA, 1.61 WHIP and 51:23 K:BB through 51 innings of work this season with Memphis.