Johnson was born in Waco, Texas. He attended Waco High School, where he was a three-sport star in football, track and basketball. In football, he played as a linebacker, registering 170 tackles (103 unassisted), 21.0 stops for loss, 6.0 quarterback sacks, five forced fumbles and two interceptions, including one returned for a touchdown as a senior. Johnson produced a career-high 30 tackles and four forced fumbles in a single game. He earned Parade magazine high school All-America and first-team All-Texas Class 5A honors following both his junior and senior seasons, and played in the first-ever U.S. Army All-American Bowl on December 30, 2000.
Also an standout track & field athlete, Johnson was a state-qualifier in the 100-meters and the triple jump. He recorded a 10.5-second 100-meter dash, while also clearing over 48 ft (14.66 m) in the triple jump. At the District 13-5A Championships, he ran a leg on the Waco 4x100m relay squad, helping them earn a fourth-place finish at 43.77 seconds.
Johnson was drafted 15th overall by the Kansas City Chiefs. After a month-long holdout by Johnson, he was signed by the Chiefs to a five-year deal worth $10.4 million. He joined Derrick Thomas and Percy Snow as the third Butkus Award winner to play for the Chiefs.
Johnson started every game for the Kansas City Chiefs in 2005, becoming the first Chiefs linebacker in 20 years to start all 16 games of his rookie campaign. He was subsequently awarded the Mack Lee Hill Award by the Chiefs (presented to the most outstanding rookie).
In Week 17 of the 2009 NFL season, Johnson had an NFL-record tying two interceptions returned for touchdowns.
November, 2010 - received a five-year contract extension. The total value of the deal was $34 million; $15 million of that in guaranteed money.
Following a stellar 2011 NFL season, Johnson was named to his first Pro Bowl and was also named 1st team All-Pro.
During a week 1 game against the Tennessee Titans on September 8, 2014, Johnson suffered a torn Achilles. Later in the day, it was announced he would miss the rest of the season. Two days later, Johnson was placed on injured reserve.