McGwire, a former USC star who lives in Orange County, rejected a contract extension and left a similar position with the Cardinals to be closer to home.
"This was one of the hardest years for me, family-wise," said McGwire, who has six children, including a grown son, 2 1/2-year-old triplets and two other sons playing Little League. "Things just worked out perfectly. I grew up a Dodgers fan living in the L.A. area. I have many memories of Dodger Stadium."
Colletti received permission from the Cards and met with McGwire on Halloween.
"When we played the Cardinals, I heard conversations from our coaches and pitchers about how difficult it is to pitch to them, because of how well they make adjustments inside the game and how well-prepared they are," said Colletti. "We're privileged and thrilled to get a person of this quality with his success and results."
The Dodgers are hoping that McGwire, with his considerable accomplishments as a hitter and coach, will connect with a lineup of veteran hitters that didn't deliver in 2012. While with the Cardinals, McGwire worked with Albert Pujols, Carlos Beltran, Matt Holliday and Lance Berkman.
"When they were in town, I saw him in the weight room and we talked about hitting approach," said manager and former hitting coach Don Mattingly. "We were the same club -- both having trouble scoring, and I could just tell he had a really good feel for what he wanted to do and how he went about it.
"It helps a little bit as a coach if you've thrown up numbers [as a player]. Guys will look you up. Hanley [Ramirez] one day said to me, 'You could hit a little bit, huh?' It gets you in the door. But if you don't know what you're doing, they'll tune you out, no matter what you did on the field. You won't get a free ride."
McGwire said his coaching style is light on mechanics and heavy on mental approach.
"I was one of the toughest, mentally-prepared players ever," said McGwire. "I will get that across. Have a game plan and be open to challenge yourself. It's all about mindset. Most players play on physical ability.
"You've got to have patience. Work the count. Get into the bullpen as soon as you can. That's what the game is all about. I can talk mechanics all day, but you can't think about mechanics in the box or you're done."
Hansen took the fall for a disappointing offense that never overcame the hamstring and shoulder injuries that hobbled the lineup's focal point: Matt Kemp. Hansen has since been hired as Seattle's hitting coach. Even second-half additions of All-Stars Ramirez, Adrian Gonzalez and Shane Victorino didn't keep the Dodgers from finishing 13th in the National League in runs scored and RBIs and 15th in home runs.
"What intrigues me is their lineup," McGwire said. "The nucleus is all under nine seasons and they're really just getting started understanding what the game is about."
When McGwire, a 12-time All-Star, was hired as Tony La Russa's hitting coach three years ago, he admitted taking performance-enhancing drugs during his career, presumably the overriding factor in his not being voted into the Hall of Fame.
"In 2010, I confronted that head-on," McGwire said. "I had to go through what I had to go through. It's something I did and I'll have to live with it the rest of my life. I understand what the Hall of Fame is all about. I regret that. It's a mistake I made, I owned up to it and moved on. I don't know what else to say."
Colletti said McGwire's positives outweigh that part of his past.
"He apologized, and that's very important," said Colletti. "We all make mistakes. He's done other things in his life of high character. He walked away from a contract instead of taking the money on the DL. You talk to people who know him, he's owned up to his mistakes. He has so many great qualities, you forget the mistakes."
With McGwire coaching, the Cardinals led the NL in batting average (.269) and on-base percentage (.337), ranked second in runs (2,263), fourth in slugging percentage (.416) and third in OPS (.753). The Cards also batted an NL-best .274 with runners in scoring position while McGwire was coach.
McGwire, who broke Roger Maris' record of 61 home runs when he hit 70 in 1998, retired as a player after the 2001 season with 583 career home runs. He currently ranks 10th all-time in the category.
McGwire was a first-round Draft pick of Oakland in 1984 and won the American League Rookie of the Year Award in '87. His ninth-inning home run beat the Dodgers in Game 3 of the '88 World Series, and he was on the A's '89 World Series championship team.
In 1997, McGwire established the Mark McGwire Foundation for Children to assist agencies that work with child-abuse victims.