The sixth-round draft pick in 2008 muddled through last year, with two calf injuries and the death of his father throwing his performance completely out of whack.
He has bounced back in a big way this season, becoming one of the few bright spots on a 2-11 team, and the Vikings rewarded him with a five-year contract extension Saturday.
The extension is worth $25 million with $10 million guaranteed, a league source told ESPN NFL Insider Adam Schefter.
"I can't thank the Vikings enough for giving me another chance," Sullivan said. "They gave me another chance to go out there and prove that last year was result of circumstances and to show what type of player I can be when I'm healthy and not dealing with some personal things."
In his fourth season in the league, Sullivan is emerging as one of the leaders on an offense that is looking for consistency and reliability. The offensive line has struggled at times this season to open holes for Adrian Peterson and keep Christian Ponder protected, but coaches say that Sullivan has quietly had a very good year.
"Game after game he's played at such a high level, been so productive for us and really become a leader in a lot of ways at the center position for the offensive line," coach Leslie Frazier said earlier this week. "He's kind of taken that mantle we need him to take on the offensive line."
Listed generously at 6-foot-4 and 301 pounds, Sullivan was initially thought to be a little undersized when he took over for longtime starter Matt Birk in 2009. But he helped the Vikings reach the NFC title game in his first season as a starter.
"Over the last four years, John has contributed in countless ways to the Vikings organization," Vikings owner Zygi Wilf said in a statement. "He has been very active in the community along with being a key member along our offensive line. John is a bright young man and we are fortunate to have him in the Vikings organization."
Now the Vikings are looking for him to take on even more of a leadership role going forward as the team tries to turn things around after this year's massive struggles on the field.
"That's a role I need to embrace and help our team with," Sullivan said. "I view my role with trying to help Christian out as much as possible, make sure he's comfortable and he can be as successful as possible."
Ponder has had his ups and downs, as expected, since taking over as the starter in Week 7. But Sullivan remains confident that the two will be exchanging snaps in Minnesota for a long time.
"He's the total package and he's the future of this team," Sullivan said. "We're going to do great things with him at the helm."
Information from The Associated Press was used in this report.