He played college football for the Utah State Aggies and was selected by the Seattle Seahawks in the second round of the 2012 NFL draft.

On March 30, 2012, Wagner attended Utah State's Pro Day and impressed scouts and team representatives after performing all of the combine drills.

[8] His 40-yard dash and broad jump would've finished first among all linebackers in at the NFL combine and his vertical would've tied with California's Mychal Kendricks for first.

[15] Wagner entered training camp competing for the vacant starting middle linebacker role against Barrett Ruud and Matt McCoy.

[18] He made his professional regular season debut and first career start in the Seahawks season-opener at the Arizona Cardinals and recorded four combined tackles during their 20–16 loss.

[19] In Week 5, Wagner recorded five combined tackles and made his first career sack on Cam Newton in the Seahawks' 16–12 victory.

[27] On January 6, 2013, Wagner started in his first career playoff game and recorded nine combined tackles during a 24–14 victory at the Washington Redskins in the National Football Conference (NFC) Wild Card Round.

[31] In Week 9, Wagner collected 11 combined tackles and was credited with 1.5 sacks on Mike Glennon during a 27–24 win over the Tampa Bay Buccaneers.

Wagner started the Seahawks' season-opener against the Green Bay Packers and recorded a season-high 14 combined tackles during a 36–16 victory on Thursday Night Football.

[40] On December 14, 2014, Wagner recorded his fourth game with at least ten combined tackles and sacked Colin Kaepernick during a 17–7 victory over the 49ers.

[45] On February 1, 2015, the Seahawks played in Super Bowl XLIX after defeating the Panthers in the Divisional Round and then the Packers in the NFC Championship.

On October 11, 2015, Wagner recorded seven combined tackles, a pass deflection and a fumble recovery, and scored his first career touchdown during a 27–24 overtime loss to the Cincinnati Bengals.

Defensive end Michael Bennett caused Bengals' running back Rex Burkhead to fumble in the third quarter, which Wagner recovered and returned it 23 yards for a touchdown.

[52] In Week 10, Wagner collected 11 combined tackles, deflected a pass, recovered a fumble, and scored his second touchdown of the season, as the Seahawks lost to the Arizona Cardinals, 39–32.

In the fourth quarter of that game, linebacker K. J. Wright sacked quarterback Carson Palmer for a nine-yard loss and jarred the ball loose resulting in a fumble.

[54] He finished the 2015 season with 114 combined tackles (67 solo) and seven pass deflections, and was credited with only half a sack in 15 games and 15 starts.

[55] Wagner and Wright returned to their respective starting roles in 2016 and were joined by Mike Morgan, who was chosen to replace Bruce Irvin following his departure in free agency to the Oakland Raiders.

[64] On January 1, 2017, Wright recorded 12 combined tackles, a pass deflection, and sacked Colin Kaepernick twice in a 25–23 victory at the San Francisco 49ers.

[76] On November 5, 2017, Wagner recorded a season-high 12 combined tackles, deflected a pass, and sacked Redskins' quarterback Kirk Cousins for the first safety of his career in the Seahawks' 17–14 loss.

[77] He finished the 2017 season with 133 combined tackles (97 solo), six pass deflections, two interceptions, 1.5 sacks, and a safety in 16 games and 16 starts.

For his first takeaway, in the second quarter, he forced a fumble from Jeffery Wilson at the Seahawk 5-yard line, recovered it at the 3, and returned it 11 yards to the 14.

Later, in the fourth quarter, he intercepted a pass from Nick Mullens at the 2-yard line and ran it back 98 yards for a touchdown, his first career pick six, to set the Seahawk franchise record for longest play.

[87] On December 10, against the Minnesota Vikings, Wagner was a part of many key third and fourth down stops as well as blocking a field goal by kicker Dan Bailey.

[99] In Week 8 against the Falcons, Wagner recorded a sack on Matt Schaub and recovered a fumble lost by Devonta Freeman in the 27–20 win.

[100] In Week 9 against the Buccaneers, Wagner recorded a team high 11 tackles and sacked Jameis Winston once in the 40–34 overtime win.

[114] In the Wild Card Round of the playoffs against the Rams, Wagner led the team with 16 tackles (11 solo) and sacked John Wolford once during the 30–20 loss.

[115] In the 2021 season, Wagner finished with one sack, 170 total tackles (93 solo), one interception, five passes defended, and one forced fumble.

[134][135] In Week 4 against the Arizona Cardinals, he surpassed Zach Thomas for fourth most career tackles in NFL history.

[136] Wagner recorded at least 100 tackles in a season for the 13th consecutive year, trailing London Fletcher for the second-longest streak in NFL history.

[139] In January 2024, Wagner began an 18-month online course from Howard University to earn a Master of Business Administration (MBA) degree.