The one-year contract will pay him $800,000 while in the major leagues and $150,000 while in the minors.
Manager Mike Shildt said LeBlanc will pitch in relief and he faced his first two batters as a Cardinal on Thursday. LeBlanc gave up a double to Freddie Freeman in the bottom of the seventh before getting Ozzie Albies out to end the inning.
LeBlanc, 36, boasts a 13-year career in the majors that saw him pitch for eight different teams. He is 34-41 with a 4.73 ERA across 121 career starts and is 12-7 with a 4.23 ERA in 125 relief appearances.
"He can pitch about anywhere, any time it sounds like," Shildt said. "Clearly we have some defined roles in close games and with the lead, so we'll just get him out there and take it from there."
LeBlanc most recently pitched for the Texas Rangers' Triple-A affiliate. He also appeared in six games for the Baltimore Orioles this season, including one start.
The Associated Press contributed to this report.