His scheduled news conference, only minutes later, was much more combative.
The second session was shifted to an overcrowded hallway at the last minute, and McGwire evaded questions about the criticism he's received from ex-players. He repeatedly emphasized that he was ready to talk about the game instead of performance-enhancing drugs.
"I hope you all can accept this," McGwire said. "Let's all move on from this. Baseball is great right now, baseball is better."
Dressed in jeans, a sweater and running shoes, the 46-year-old McGwire walked on stage to "Welcome to the Jungle" by Guns N' Roses, the hard-rock song played before his at-bats with the Cardinals. The team's new hitting coach was cheered by fans who secured seats as much as 3½ hours earlier.
"I've learned a lot," McGwire told fans. "Especially to kids out there, steroids are bad. I made a huge mistake in my life and it's something I want you guys to learn from. Don't ever, ever go down that road."
Jessica and Sarah Schaaf were in the front row of a downtown hotel ballroom jammed with perhaps 1,000 fans, and wore T-shirts made for the occasion that said "Welcome back, Big Mac Land," with a photograph of McGwire.
"He did wrong," Jessica Schaaf said. "But we still love him."
In a brief appearance on stage, McGwire said he was happy about his chance to put on a major league uniform again. The former home run king headed over to Busch Stadium, just blocks away, for an afternoon hitting session with Colby Rasmus and Ryan Ludwick, and pledged to immerse himself in his new job.
"Like I told them, I'll be the first one in the cage and I'll be the last one to leave," McGwire said. "I'm there for them, I'm there to pass on my knowledge."
McGwire, hired in October, added he had a "huge Rolodex of knowledge" to pass on to Cardinals hitters.
He was supposed to be at a podium minutes later, but when his news conference was moved into a narrow hallway it left reporters jostling for space and shouting questions. The session lasted just more than six minutes before questions were cut off and McGwire was escorted out by security and police through a back door.
A team official called the last-minute switch an "executive decision."
McGwire said he's been "dead honest" in interviews since the admission. But he wasn't interested in rebutting criticism from former Oakland teammate Jose Canseco, who said McGwire is still lying by denying that the two players injected themselves with steroids in clubhouse bathroom stalls.
"I'm not going down that road with Jose," McGwire said. "I'll take the high road with the Jose stuff."
Former Cardinals slugger Jack Clark, who called McGwire a "phony" in a St. Louis Post-Dispatch story last week, was booed in an appearance on the same stage not long before McGwire's appearance.
"I heard he said something, I don't know what he said," McGwire said. "Hey, listen, they have their opinions. I was being as honest as I am."
McGwire denied again that Tony La Russa, his manager virtually all of his career, had any knowledge of his steroids use until the day he went public.
"I kept this to myself," McGwire said. "You know what? I spoke from my heart. I hope you all can accept this, let's all move on from this."
La Russa is ready for the day steroids ceases to be an issue, telling fans that McGwire deserves another chance.
"I'm just here to tell you this is a really good man," La Russa said.
Teammates have consistently been supportive, too. Cardinals slugger Matt Holliday, who has hit with McGwire in prior offseasons, although not this one, said he's never brought up the steroids topic.
"It's not a great situation to be in, but he's dealing with it like a man," Holliday said. "He doesn't have to explain anything to me."
For his part, though, Holliday said he has no interest in performance-enhancing drugs, to the point where he's stopped taking vitamin supplements.
"I'm not willing to take risks," Holliday said. "I'll stick to like a protein shake and that's about it."
General manager John Mozeliak is among those skeptical of McGwire's assertion that he would have been a home run champ even without steroids, but said the team didn't want to coach McGwire on what to say.
"I think that's what he believes," Mozeliak said. "I think when you look at the steroid correlations of strength and speed and what science has proven to date, it has to be factored in.
"He is recognizing it did allow him to be on the field, which obviously would create more opportunities."