Sullivan went into the 2008 NFL Draft as a fifth-year senior. Although his physical attributes were less than many in his position, he was rated as the fourth-best center in the draft, projected to be picked in the third or fourth round. The first four rounds passed with fellow centers Mike Pollak, Cody Wallace and Kory Lichtensteiger all selected ahead of Sullivan. He was eventually selected by the Minnesota Vikings with the 21st pick of the sixth round (187th overall).
After spending a year as backup to starting center Matt Birk, Sullivan was promoted to the starting position after Birk joined the Baltimore Ravens as a free agent in 2009. That year, he started every game as the Vikings and their new quarterback Brett Favre improved their offensive ranking to #5 in the league (#8 in passing). The following year, he struggled with a calf injury and missed back-to-back games away to the New York Jets and at home to the Dallas Cowboys, with Ryan Cook and Jon Cooper respectively filling in. He missed one more game in 2011, a Week 7 matchup against the Green Bay Packers in which Joe Berger took his place. As a reward for his efforts, Sullivan was given a five-year, $25 million contract extension by the Vikings on December 17, 2011.
The following season, he anchored the Vikings' offensive line as they provided the platform for RB Adrian Peterson to reach 2,097 rushing yards for the season, eight yards short of the single-season record held by Eric Dickerson. His performances throughout the season saw him named in the Pro Football Writers Association's All-Pro team; nevertheless, he missed the cut for the Pro Bowl, the center spot going to Max Unger of the Seattle Seahawks. Following the Vikings' elimination from the playoffs, Sullivan underwent surgery on microfractures in his left knee, expected to keep him sidelined for most of the offseason.