Mankins, who wanted a contract extension, had stayed away since deciding not to sign the team's tender offer by June 14, the day before minicamp began.
"It's good to have him back on the team," said coach Bill Belichick, who met with Mankins on Tuesday morning. "He is a part of the team here now."
The original tender offer of $3.26 million was reduced to $1.54 million when he didn't sign. The team did not announce how much Mankins would be paid for the rest of the season.
The Patriots (6-1), who visit the Cleveland Browns on Sunday, were granted a roster exemption until Monday for Mankins, NFL Network insider Jason La Canfora reports, citing a league source. They would then have to activate him and make a roster move.
Mankins, chosen for the Pro Bowl in 2007 and 2009, completed his original five-year contract last season and became a restricted free agent. He had said he was told the Patriots would make him the highest-paid guard in the NFL, a promise the team denied making. He also said he wanted to be traded.
"The guys are jumping for joy," Patriots running back coach Ivan Fears said after Mankins showed up. "Having another playmaker like him coming back to play for us? Sure. Plus he's fresh. We're kind of excited about it, no doubt about it. We can't wait to see where he's at out on the practice field."
Players return to practice Wednesday after Tuesday's day off.
Dan Connolly has started at left guard on a line that has allowed just 12 sacks. In his first three seasons with the Patriots, Connolly started four of the 15 games in which he played. He joined the NFL in 2005 with Jacksonville as an undrafted rookie and was signed by the Patriots in 2007, spending the entire season on the practice squad.
His playing time is likely to be reduced, especially after he was replaced by Ryan Wendell in the second half of a 23-20 win Oct. 24 at San Diego.
"We've had a lot of players that have played well for us to this point," player personnel director Nick Caserio said, "but the more good players you have in your program, the more good players you can get on the field."
The Patriots drafted Mankins with the 23rd pick of the first round in 2005 and he started all 80 regular-season games and nine playoff games until this season.
"He's been a great player and person for our football team," Belichick said. "He is tough. He's smart. He's a dependable player, and he's done all those things since he has been here. You just can't have too many guys like that on your football team, so we're glad he's back."
Had Mankins not reported by Nov. 16, he would have lost a year toward free-agency eligibility.
Under the most recent collective-bargaining agreement, a player could become an unrestricted free agent after five seasons. That increased to six when owners opted out of the deal. Unless Mankins signs beyond this season, he can become an unrestricted free agent.
His holdout began with criticism of the organization.
ESPN Boston had reported that Mankins and his agent, Frank Bauer, asked the Patriots after the 2008 season about a contract extension but were told that owner Robert Kraft wanted to wait because of the uncertain labor situation.
"I was asked to play '09 out and that they would address the contract during the uncapped year. I'm a team player, I took them at their word, and I felt I played out an undervalued contract," Mankins said in June. "Right now, this is about principle with me and keeping your word and how you treat people. This is what I thought the foundation of the Patriots was built on. Apparently, I was wrong. Growing up, I was taught a man's word is his bond. Obviously, this isn't the case with the Patriots."
He later apologized.
Asked if he would take Mankins' criticism into account in deciding when to use him, Belichick said: "As always, we will do what's best for the football team."
Caserio wouldn't say if the Patriots expect to work out a long-term deal.
"Those discussions will remain between ourselves and the player and his representatives, like they always do with every player," he said.
The Associated Press contributed to this report.