McGwire issued a statement late Sunday, saying he did not feel his physical condition would allow him to play his best anymore.
Cardinals spokesman Brian Bartow said the team plans no formal announcement.
"All indications point to the fact that what we received from Mark is accurate," Bartow said.
Bartow added that general manager Walt Jocketty is trying to contact McGwire, who is vacationing in Mexico.
"I am walking away from the game that has provided me opportunities, experiences, memories and friendships to fill 10 lifetimes," McGwire said in a statement faxed to ESPN.
McGwire, who has been bothered by a sore right knee for the past two years and back problems, hit .187 in an injury-plagued season this year but manager Tony La Russa said recently he thought McGwire had some playing time left.
In 1998, McGwire broke Roger Maris' single-season home-run record, hitting a record 70 roundtrippers in a slugging duel with Chicago Cub Sammy Sosa. That new mark was broken this year by San Francisco's Barry Bonds, who hit 73.
McGwire first indicated he was thinking of retiring in July and never signed the $30 million, two-year contract offered him. McGwire earned $11 million this year, the last year of a four-year deal.
La Russa told the St. Louis Post-Dispatch he understands McGwire's frustration.
"But who knows what he goes through better than him?" La Russa asked. "Nobody deals with the frustration he does. Nobody deals with the rehabilitation and nobody deals with the expectations he does."
McGwire hit 29 home runs this season, after spending time on the disabled list in April following surgery to repair a diseased tendon.
In the first round of playoffs, McGwire went one for 11 with six strikeouts and La Russa sent in rookie Kerry Robinson to pinch hit in the ninth inning in Game 5.
"One of my lowest moments as a manager was pinch-hitting for Mark," La Russa said. "But you've got to try to win the game.
"Pinch-hitting for Mark is something I'm not going to remember fondly."
McGwire posted 583 career homers and a career batting average of .263. Before coming to the Cardinals in 1997, McGwire played for 12 years in Oakland.