The Chiefs announced the deal Wednesday night.
The two-year extension runs through the 2017 season, a person familiar with the contract said on condition of anonymity because the team didn't disclose terms of the contract.
The six-year NFL veteran ran for 1,287 yards and 12 touchdowns last year to help the Chiefs reach the playoffs. He also had 70 catches for 693 yards and seven more TDs as Kansas City went 11-5 before losing to Indianapolis in an AFC wild-card game.
''Jamaal is an elite player in the National Football League,'' Chiefs General Manager John Dorsey said in a statement released by the team. ''It was important for us to keep him here in Kansas City long-term.''
Charles was expected to take part in the first practice scheduled for Thursday.
''I had no intention on holding out,'' Charles tweeted. ''I just ran out of gas on the way to camp and my cellphone battery died. It was a long walk I tell ya.''
Charles comes off a season with career highs in yards from scrimmage (1,980), rushing touchdowns, total touchdowns, receptions, receiving yards and receiving touchdowns.
Charles' earned a base salary of $1.75 million in 2013, according to NFLPA records. He was set to earn a base salary of $2.65 million, which ranked 11th among NFL running backs, before the contract extension. Various media reports, including from NFL.com and ESPN, put Charles' two-year extension at $18 million.
With Charles secured through 2017, the Chiefs can shift focus to quarterback Alex Smith and outside linebacker Justin Houston, both of whom enter the final years of respective contracts.
Smith, 30, comes off a season where he completed 308-of-508 passes for 3,313 yards and 23 touchdowns against 13 interceptions. He established career highs in rushing attempts (70) and yards (431), and became the second quarterback in team history to start the season with nine consecutive wins.
Smith reported Sunday with rookies and quarterbacks and put in two morning practices before being excused from Wednesday's practice. The Chiefs' second-year quarterback attended voluntary organized activities and mandatory minicamp.
The same can't be said of Houston, who missed the Chiefs' offseason workout program. Houston is expected to report for training camp, however.
''From what I hear, he's going to be here,'' outside linebacker Tamba Hali said. ''We're going to get it rocking again - Sack City. Whatever happened in the offseason, we put all of that behind us. He's here to play football just like every other man. That's our concern - get to that quarterback.''
Houston, 25, comes off his second straight Pro Bowl selection and finished the season with 11 sacks in 11 games. He and Hali combined for 22 sacks in 2013.
Houston is entering the final year of his rookie contract, which pays him a base salary of $1.4 million. His 26.5 sacks since 2011 matches Green Bay Packers outside linebacker Clay Matthews' sack production during that span.
AP Sports Writer Dave Skretta and AP freelance writer Herbie Teope contributed to this report.
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