He played college football for the Penn State Nittany Lions, where he won the Maxwell Award and was a unanimous All-American in 2002.

He was selected by the Kansas City Chiefs in the first round of the 2003 NFL draft, and also played for the Cincinnati Bengals, the Washington Redskins, and the Miami Dolphins.

Johnson was chosen in the first round with the 27th overall pick of the 2003 NFL draft as insurance for the Kansas City Chiefs, who were unsure if Priest Holmes would be healthy or even sign a contract extension.

[6] Johnson was drafted over the objection of head coach Dick Vermeil, who wanted to select a defensive player,[7] and despite the lack of recent NFL success by Penn State running backs (Blair Thomas, Ki-Jana Carter, and Curtis Enis).

The Chiefs made an appearance in the playoffs with a 9–7 record, where Johnson ran for 32 yards on 13 carries against the Indianapolis Colts.

[10] As a result of the extension, Johnson was the highest-paid running back in the NFL based on average salary per year.

[citation needed] In week 9 of the 2007 regular season, Johnson was sidelined late in the 4th quarter against the Green Bay Packers with a foot injury.

The injury was season-ending; Johnson did not see any playing time in the rest of the 2007 season, and was placed on the injured reserve list.

Johnson, after weeks of being inactive for the Chiefs, was suspended by league commissioner Roger Goodell for the team's week 10 game against the San Diego Chargers, for violating the league's personal conduct policy, after he was charged with simple assault for allegedly spitting his drink in a woman's face (the fourth time in five years he had been accused of assaulting a woman), and as he awaited a court date for allegedly pushing another woman's face.

[22] At the time of the suspension, Johnson was only 75 yards from passing Priest Holmes as the franchise's all-time leading rusher.

Reportedly, the final straw for Chiefs general manager Scott Pioli was yet another tweet in which Johnson belittled a fan for making less money than he made.

Johnson signed a three-year contract worth up to $12 million with the Washington Redskins as an unrestricted free agent on March 12, 2010.

Johnson appeared on the cover of the PlayStation 2 game NCAA GameBreaker 2004 in a Penn State uniform.

In October 2013, TMZ reported that Johnson had been a recurring guest DJ at Miami strip club Tootsie's Cabaret.

[29] Because of memory loss, suicidal impulses, mood swings, and headaches, Johnson believes he has chronic traumatic encephalopathy (which can't be diagnosed until an autopsy is performed on the brain after death).

[31] His method of dealing with the trauma is to control his social interactions and spend quality time with his daughter.

[32] Since 2020, Johnson has been criticized for posts on his Twitter account including antisemitic comments such as claiming Jews worship Satan, and attributing fake quotes to the Bible.

[33] CNN reporter Jake Tapper called his comments explicitly "antisemitic garbage",[34][35] due to references to a "jewish cabal" and posting alleged quotes regarding "jews toy(ing) with the public as a cat toys with a mouse" or worshipping [satan].

"[36] Johnson's tweets have also touched on topics common among conspiracy theory proponents, including freemasonry.

[37] He frequently tweets conspiracy theories about celebrity deaths, including the death of Kobe Bryant, which he claimed that LeBron James made a "blood sacrifice" to kill Bryant in exchange for the Los Angeles Lakers winning the 2020 NBA Finals.

[40] In December 2003, he was arrested for felony aggravated assault and misdemeanor domestic battery for slapping and waving a gun at his then-girlfriend as he threatened her, during an argument at his home.

[43] He was required to complete 120 hours of community service, attend an anger management course, and stay out of trouble for two years.

In March 2009, Johnson pleaded guilty to two counts of disturbing the peace, regarding the two incidents in 2008, and was sentenced to two years' probation.

[43] He was booked into the Clark County Detention Center with bond set at $15,000, and was released after spending 18 hours behind bars.

[56] However, a felony charge was dropped as part of a plea deal, and he was ruled guilty of misdemeanor domestic violence battery and assault, and sentenced to one year of probation and ordered to pay a $345 fine, perform 48 hours of community service, and attend six months of counseling.

[53][57] On October 7, 2014, Johnson was arrested for the sixth time after punching a 29-year-old man in a Miami Beach club, and allegedly cutting him with a broken bottle.

Johnson later pleaded guilty to battery and was sentenced to 12 months' probation, 60 hours of community service, and must submit to substance abuse evaluations, and stay away from the victim and the club.