Esteban Loaiza and Orlando Cabrera also switched teams in two of the eight trades that came less than an hour before the 1 p.m. PDT deadline.
"My initial reaction was, 'Wow,' " Garciaparra said.
With more than 20 teams still in the playoff hunt, a lot of clubs were eager for action. AL West-leading Texas nearly got Larry Walker, but the Colorado slugger turned down a trade.
But those old October heartbreakers, the Red Sox and Cubs, got busy.
Garciaparra was already wearing his Boston jersey when Manager Terry Francona called a team meeting before the game at Minnesota.
The five-time All-Star shortstop and two-time AL batting champion was headed to the Cubs.
"If it was in my control, I'd still be wearing a Red Sox uniform, because it's the place I know, I love. All of those fans, I'll always remember," Garciaparra said.
"But I'm also going to another great place. I'm going to a phenomenal city with great tradition as well, phenomenal fans, great organization."
And he added: "Hopefully, we'll see them in the World Series."
Cub General Manager Jim Hendry was ecstatic to get Garciaparra, in the last year of his contract and the subject of trade rumors all year.
"I think he will bring a ton to the table and a presence on the field and off," Hendry said. "You never go to work thinking he's going to be available."
Garciaparra, 31, is batting .321 with five home runs and 21 RBIs in 38 games this season. He missed the first 57 games with an injured Achilles' tendon.
The Red Sox wound up with Cabrera, Montreal's Gold Glove shortstop, and Minnesota first baseman Doug Mientkiewicz, another Gold Glove winner.
The Cubs got Garciaparra and minor league outfielder Matt Murton. Montreal acquired Cub shortstop Alex Gonzalez, pitcher Francis Beltran and infielder Brendan Harris and the Twins got minor league pitcher Justin Jones.
After getting shut out on Johnson, the AL East-leading Yankees swapped the inconsistent Contreras and cash for Loaiza, a 21-game winner last season.
Loaiza is 9-5 with a 4.86 ERA in 21 starts this year and made his second straight All-Star appearance. He is eligible for free agency after this season.
"It's kind of hard for me right now, especially all the good stuff I've had in Chicago and the success I've had here," Loaiza said, "but the Yankees are the Yankees."
Contreras is 8-5 with a 5.64 ERA in 18 games. The top pitcher in Cuba when he defected, his up-and-down tendencies perplexed and frustrated the Yankees for two years.
In other trades:
* Florida acquired starting pitcher Ismael Valdez and reliever Rudy Seanez. The World Series champions sent minor league pitcher Travis Chick to San Diego for Valdez and dealt outfielder Abraham Nunez to Kansas City for Seanez.
* Texas got pitcher Scott Erickson and cash from the New York Mets for a player to be named.
Starting late Saturday afternoon, teams can still make trades, but it becomes a more tricky process. Players must pass through waivers -- meaning any other club can claim them -- before a deal.
Teams have until Aug. 31 to acquire players in order for them to be eligible for the postseason.