Johnson recorded 29 tackles at linebacker in his freshman season for the Kentucky Wildcats. He was voted to the All-SEC Freshman Team by the coaches of the conference. In the Wildcats' 2006 Music City Bowl victory against Clemson University, Johnson entered the game as a running back and scored a touchdown. As a sophomore, he recorded 58 tackles, 2 interceptions, and 5 pass breakups. As a junior, he recorded 57 solo tackles, 30 assisted, 11.5 for loss, and 2.5 sacks. Johnson earned first-team All-SEC honors, and he considered early entry into the 2009 NFL Draft. However, Johnson opted to stay for his senior year.
On December 8, 2010, Johnson was signed to the practice squad by the Kansas City Chiefs. He was waived on September 3, 2011.
Cincinnati Bengals
On December 13, 2011, Johnson was signed by the Cincinnati Bengals to the practice squad.
Calgary Stampeders
On May 27, Johnson was signed by the Calgary Stampeders of the Canadian Football League. Johnson worked his way up the ranks with Calgary over the years, winning the Grey Cup in 2014 as a backup. Subsequent years saw Johnson working his way into a starting role, while returning to the championship game for the 2016 and 2017 seasons, where Calgary suffered a pair of heartbreaking losses to Ottawa and Toronto. In 2018, Johnson and the Stamps won the 106th Grey Cup, and Johnson's efforts resulted in his recognition as one of the most dominant interior rushers in the game, with 14 sacks, 3 forced fumbles, and an interception. He was named to his 3rd consecutive CFL-West All Star team. In 81 career games during his 6 seasons in Calgary, Johnson has amassed 158 tackles, 2 tackles on special teams, 41 sacks, 2 interceptions, and 6 forced fumbles.
Saskatchewan Roughriders
On February 12, 2019, Johnson signed a one-year contract with the Saskatchewan Roughriders for $250,000 CAD, making him one of the highest paid defensive players in the CFL. Johnson missed a few games with injury, and had a slow start to the year while adjusting to the new team and defensive scheme; Johnson did not record his first sack until week 13 during the 2019 edition of the Banjo Bowl. However, it was the beginning of a critical stretch of games for the Riders in which Johnson had three consecutive games with a sack, with four in five games. Johnson's 26 tackles and four sacks on the year helped the Riders finish with the best record in the West Division.
BC Lions
Upon entering free agency, Johnson signed a one-year contract with the BC Lions on February 11, 2020.
