McGwire becomes rookie manager Andy Green's right-hand man in the dugout after spending the last three seasons as hitting coach of the rival Los Angeles Dodgers. Before that, McGwire was hitting coach of the St. Louis Cardinals for three seasons following a hiatus after his steroid-tainted playing career ended.
Big Mac said during a conference call Wednesday that he brings "a lifetime of experience" to his new job.
"As a player I pretty much went through anything a player could ever wish upon as far as success, and all the failures," he said. "You learn how to be a man through adversity and failure. The game is built on failure. I know how to bust through that. ... I'm ecstatic about this new role. It's a great challenge for me, moving up in the game."
McGwire, who lives up the freeway in Irvine, said he hit it off right away with Green and "was quite flattered when he offered me this job."
Green said he had no previous baseball relationship with McGwire but got to know him well during the last few weeks.
"This guy has everything I'm looking for in the bench coach role," Green said. "He demonstrated it time and time again."
Green said he could have gone for a candidate who had previously managed in the big leagues, "but I wanted someone who understood the game and understood human beings, and I believe he has that."
The manager said everyone he spoke to about McGwire had the same "No. 1 distinguisher, which is that he has the ability to connect with all players of every background. I loved the willingness to talk about a difficult road. ... I think players respond to a man who hit 583 home runs but talks about adversity in the game."
As far as moving from being a hitting coach to being a bench coach, "It's a new role, a learning experience," McGwire said. "I'm all eyes and ears. It's a new team. It's the same thing when I got back into the game in 2010."
In 2010, McGwire admitted he used steroids when he broke baseball's single-season home run record by hitting 70 in 1998. Even so, McGwire asserted he would have hit home runs without performance-enhancing drugs.
McGwire is on the baseball writers' ballot for the Hall of Fame for the 10th and final time this year. Last year he received his lowest vote total, yet, 10 percent. He's never been named on more than 23.7 percent of the ballots since he's became eligible in 2007.
"I believe this is my last year so I don't think the tide's going to turn," he said.
Also new to the Padres are hitting coach Alan Zinter, first base coach Tarrik Brock, bullpen coach Doug Bochtler and Eddie Rodriguez, who will serve in a variety of roles.
Returning are pitching coach Darren Balsley and third base coach Glenn Hoffman.
Green replaced interim manager Pat Murphy, who in turn replaced Bud Black, who was fired in mid-June.