Axford, who hails from Port Dover, Ont., was traded by Milwaukee on Friday in exchange for a player to be named later.
The 30-year-old relief pitcher is expected to join the Cardinals Friday night when they begin a seven-game road trip in Pittsburgh.
St. Louis general manager John Mozeliak calls Axford "a reliever that offers experience as both a setup man and closer. He's also a pitcher who brings familiarity with the NL Central, pennant-race baseball and the post-season."
However, the hard-throwing Axford has been inconsistent since his breakout 2011 season, posting a 4.57 earned-run average over 137 innings pitched.
This season, Axford has a 6-7 record, 4.45 ERA and 54 strikeouts in 54 2/3 innings. But he lost the closer's job to fellow Canadian Jim Henderson earlier in the season after failing to convert six opportunites and opposing hitters are batting .286 against him.
Axford, who has struck out 147 over 124 innings, could give the Cardinals a right-hander to pair with lefty Kevin Siegrist in the seventh inning ahead of set-up man Trevor Rosenthal and closer Edward Mujica.
Axford emerged late in the 2010 season, supplanting future Hall of Famer Trevor Hoffman as the Brewers closer. The 30-year-old quickly won over Milwaukee with his blazing fastball and befuddled opposing hitters in 2011, converting 46 saves, including a franchise record 43 in a row, as the Brewers won their first division title since 1982.
But the magic disappeared, almost suddenly, a year ago. Axford lost his command and during one painful mid-season stretch was removed from the closer's role.
By August, Axford returned to form and closed out the season converting 17 of his last 18 save opportunities and finished with 35 on the season while going 5-8 with a 4.67 ERA.
But he struggled out of the gate in 2013, allowing nine runs in his first four outings.
The Brewers signed the six-foot-five, 195-pound Axford as a free agent in March 2008 and assigned him to Brevard County of the Florida State League, where he sported a 4.55 ERA in 26 games, including 14 starts.
Milwaukee made Axford a full-time reliever in 2009 and it paid off as he fashioned a 1.63 ERA in 19 appearances and fanned 43 batters in 27 2/3 innings before earning promotions to AA Huntsville and AAA Nashville later that season.
Boasting a 97 mile-per-hour fastball, Axford collected eight victories and converted 24 of 27 save chances in 50 games with the Brewers in 2010. He also had a 2.48 ERA, struck out 76 batters in 58 innings pitched and held opposing batters to a .201 average.