Chalmers attended Kansas University and was a standout player. During the 2006-7 season, Chalmers was named the Co-Defensive Player of the Year, the All Big 12-Third Team, the All-Big 12 Defensive Team, and was the Big 12 Tournament Most Outstanding Player.

He was named Big 12 All-Defensive First Team and All-Big 12 Second Team in 2007-8. Chalmers helped his team win the 2008 NCAA Championship, and was named Most Outstanding Player of the tournament. He was also named to the NCAA's All-Defensive First Team.

Chalmers was drafted in 2008 by the Minnesota Timberwolves. The rights to Chalmers were later included in a trade with the Miami Heat.

On November 5, 2008 Chalmers set a new Miami Heat team record for steals with 9 in his fourth game in the NBA. Chalmers is on track to reach the Rookie Season Steal Record set by Dudley Bradley (Indiana, 1979-80) with 211 steals for the season.

On December 9, 2011, Chalmers officially re-signed with the Heat on a 3 year deal. He was a member of the 2011-2012 NBA Champion Miami Heat.