La Russa agreed to return for a 15th year as manager Monday with a one-year contract, the first time he hasn't had a multiyear deal with the team.
All of his coaches will return except for Hal McRae, who will be replaced by McGwire.
"Mark is passionate about the game, passionate about the Cardinals," chairman Bill DeWitt Jr. said. "Tony thinks he'll be a great coach, and I think he's got a lot to offer."
McGwire was not at the news conference at Busch Stadium, but La Russa and general manager John Mozeliak said there will be no effort to shield McGwire from questions about steroids. The team anticipated a telephone news conference with McGwire.
"By no means is he trying to hide, and by no means are we trying to hide him," Mozeliak said.
McGwire hit a then-record 70 home runs in 1998 and retired with 583 homers and a .263 career average in 2001.
He famously refused to answer questions about steroids use during a March 2005 congressional hearing, saying he wasn't there to talk about the past.
Former congressman Tom Davis, who led the committee that held the hearings, told the Daily News of New York that he welcomed the news.
"I think he's entitled to another chance," Davis told the newspaper.
McGwire has received just under 25 percent support from voters in his three appearances on the Hall of Fame ballot, well under the 75 percent needed. La Russa hopes the 46-year-old can use this position to repair a tarnished image.
"I'm a big fan of his," the manager said. "He's back in uniform and, hopefully, people will see his greatness. But the No. 1 reason he's here is to coach our hitters."
La Russa said he first contacted McGwire about the coaching job a week earlier, when he was still supposedly mulling his future. He said McGwire showed immediate interest, but the manager said the steroids topic was not broached.
"It's up to Mark how he wants to handle it," La Russa said. "What we want him to do is coach our hitters, and if he does that well, we're going to be happy."
La Russa had been attempting to persuade McGwire to return to the team as a spring training instructor for several seasons. La Russa managed McGwire for all but one of his 16 seasons, both in Oakland and St. Louis.
McGwire has worked with Cardinals hitters and other major leaguers in recent offseasons at his California home.
The group includes Matt Holliday, which could make McGwire an asset in the team's negotiations with the free-agent outfielder.
Cardinals hitters such as leadoff man Skip Schumaker have long been complimentary of McGwire's tutoring, and La Russa said he has witnessed McGwire's ability to pass on hitting knowledge.
La Russa said McGwire always refused the Cardinals' previous offers because his family was younger.
"I watched some sessions and I said, 'Wow, he really has a good approach,' " La Russa said. "He's got the whole thing -- mechanics, the power of the mind. So I asked him."
La Russa, who turned 65 on Oct. 4, had been assessing his interest in returning since the NL Central champion Cardinals were swept in the division series by the Los Angeles Dodgers on Oct. 10. His $8.5 million, two-year contract was to expire at the end of the month, and he said he finalized his decision last weekend.
"I would be here if Mark wasn't the coach," La Russa said. "The fact that he's here adds some more fire to the gut."
La Russa has produced eight playoff teams and the Cardinals have drawn 3 million-plus fans at home in 11 of his 14 seasons. He and DeWitt both said the one-year contract is better because La Russa reassesses his interest every year, and not because the manager is looking at 2010 as a farewell.
"I know there's an end in sight somewhere," La Russa said. "This is a great place to be. Past stars show up all the time. You've got to be a mummy not to be affected by that."
McRae was hitting coach for five seasons, but the team struggled offensively toward the end of the season and in the playoffs. The Cardinals hit .133 (4-for-30) with runners in scoring position in the postseason.
Pitching coach Dave Duncan also will return. He became upset with the organization, fans and reporters when his son, outfielder Chris Duncan, was traded to Boston in July. The Red Sox released Chris Duncan soon after the trade.
"Where he coaches, that's where I want to manage," La Russa said. "He's that good."