He was once was a star—a big fish in the small pond of early-2000's Kansas City. Playing for an average team and running behind a very average O-line, he was still able to grind out the yards with enough frequency to earn himself two Pro Bowl slots and the sort of reputation that makes defensive coordinators nervous.
Then, in 2009, it all went very wrong. Johnson wasn't the first player to land himself in trouble over comments made on Twitter, and he certainly won't be the last, but his fall from grace as a result was startling.
Johnson first made his offensive comments about coach Todd Haley to a reporter, but then made the mistake of both repeating them on Twitter and then using the same epithets when a fan challenged him over them. You can read a very succinct precis of the story here.
Inevitably, Johnson found himself cut by the Chiefs. His season wasn't going that well anyway, as over 370 yards of carries had resulted in no touchdowns and two fumbles. Suddenly, though, he found himself something of a pariah. No one really wanted him.
He was picked up by the Bengals, who added him to Cedric Benson in their stable of interesting running backs. He didn't start any of the seven games he played before being cut by them.
In 2010 he had a brief spell with the Redskins, managing to net only two yards in two games.
This could well be as far as it goes for Johnson, at least in the NFL. He's 31 now, and has had eight seasons of his body being pounded as only a running back's body can. He hasn't scored a touchdown since December 21, 2008.
More significantly, he couldn't break into a four-man running back rotation which now includes Reggie Bush, two rookies and Lex Hilliard, a player who didn't carry a ball in 2010 and has just one touchdown in three NFL seasons.
Taking all of this into consideration, it could just be that Johnson has had the first NFL career to die of shame.