Johnson, a 21-year big league veteran who spent the past two seasons with the Arizona Diamondbacks, was born in Walnut Creek, Calif., about 30 minutes from the Giants' waterfront ballpark. He grew up in nearby Livermore.
The five-time Cy Young Award winner has 295 victories after going 11-10 with a 3.91 ERA in 30 starts last season. He can earn an additional $5 million in performance bonuses.
The Giants offered several things on Johnson's wish list: spring training in the Phoenix area, and a chance to stay on the West Coast and in the NL West so he can pitch near his current home in Arizona.
"All of those things kind of fell into place with the Giants," Johnson's agent, Barry Meister, said in a telephone interview.
Johnson joins fellow Cy Young winners Tim Lincecum (2008) and Barry Zito (2002) in an intriguing rotation that also features promising right-hander Matt Cain. San Francisco becomes the first team with three Cy Young Award recipients since the 2002 Atlanta Braves with Tom Glavine, Greg Maddux and John Smoltz.
"He's looking forward to pitching between Lincecum and Cain and serving as a mentor for the young pitching staff," Meister said.
Johnson has 4,789 strikeouts, second on the career list to Nolan Ryan (5,714). The 6-foot-10 lefty made $16 million last season, when he struck out 173 and walked 44.
The Oakland Athletics were among the teams interested in Johnson, a 10-time All-Star who filed for free agency last month. He and the Diamondbacks had serious discussions about a new deal that could have kept him in Arizona, but the sides failed to reach an agreement.
During the winter meetings this month in Las Vegas, Giants general manager Brian Sabean didn't hide the fact that he was seeking a veteran starting pitcher for a short-term deal and that Johnson would be a great fit. Left-hander Noah Lowry is a question mark after undergoing two operations this year that sidelined him all season, one for a nerve problem in his forearm and then an arthroscopic procedure after the season on the back of his pitching elbow to remove bone spurs