Back from a trip to Japan, Valentine is replacing Terry Francona, who asked the Red Sox not to pick up the options on his contract.
Valentine's hire will mark the end of a protracted search for Francona's successor.
Following an epic collapse down the stretch, Francona parted ways with the Red Sox on Sept. 30, ending an eight-year tenure that included two World Series championships.
Shortly after Francona left, general manager Theo Epstein was also in the process of moving to the Cubs, which put the managerial search on hold for a bit.
Ben Cherington succeeded Epstein as GM on Oct. 25 and spent his first month on the job trying to identify the best manager for the Red Sox going forward.
Valentine was one of six men -- including finalist Tigers third-base coach Gene Lamont -- who interviewed for the job, and easily the most experienced of the group.
For a while, Dale Sveum was portrayed as the front-runner. But after a second interview on Nov. 16 that included Red Sox ownership, no offer was made, and the Cubs, led by Epstein, hired Sveum.
It was after Sveum went to the Cubs that rumors started to swirl about Valentine as a candidate. In truth, Cherington and Red Sox president/CEO Larry Lucchino had initially met with Valentine at a charity event in Connecticut on Nov. 3, but initially kept his candidacy under wraps to avoid any conflict of interest due to his job at ESPN.
Valentine had a second interview at Fenway Park on Nov. 21 and held an informal session with the Boston media afterward.
The 61-year-old Valentine last managed in the Majors for the Mets in 2002 but jumped at the chance to interview for the Boston job.
"Well, other than they have one of the best teams in baseball, one of the best organizations in baseball, one of the greatest places and venues in baseball, with a great, now, winning tradition over the last 10 years, other than that, there's really no reason that I want to be here," Valentine quipped after his second interview.
That same day, Cherington seemed intrigued by what Valentine could offer the Red Sox.
"Highly intelligent, creative, open-minded, certainly experienced, [he] has won titles in Japan, he's won in a major market and has a real passion for the game," Cherington said on Nov 21.
This will be Valentine's third stint as a Major League manager.
He managed the Rangers from 1986-92, going 581-605, and the Mets from 1996-02, notching a 536-467 record. That stint included a trip to the National League Championship Series in '99 and a World Series berth a year later, where the Mets lost a five-game Subway Series to the Yankees.
He also went to Japan, managing the Chiba Lotte Marines (2003-09).
Valentine is known for being a savvy tactician and having a great baseball mind. He has also had his share of controversy over the years and didn't hide from that during his recent meeting with the Boston press.
"If only all your experiences could be good then we'd live in this fairly land that Fenway Park is built around. You can't. I've had bad experiences that I hope I learn from and I've had good experiences that I hope I've learned from," Valentine said.
"Some of those bad experiences, I think I caused. Some of them were caused by the surroundings. Some of the good experiences I had, I had something to do with them, and some of them I was just happy to go along for the ride. I hope like [heck] I've learned from whatever experiences I've had."