In the 2006–2007 Big 12 Coaches Awards, Chalmers was named Co-Defensive Player of the Year with Marcus Dove of Oklahoma State. Chalmers was also voted on the All Big 12 Third Team, and the All Big 12 Defensive Team.
In the 2007–2008 season, Chalmers was named Big 12 All-Defensive First Team and All Big 12 Second Team. He was also named Most Outstanding Player of the 2008 NCAA Championship.
Chalmers attended Bartlett High School in Anchorage, Alaska. Chalmers was named 4A State Player of the Year three times in a row, joining Trajan Langdon as the only other player to do so. Chalmers also led his school to two state championships in 2002 and 2003, and a runner up finish in 2004.
Chalmers was rated the #2 overall point guard in the 2005 recruiting class by Rivals.com and Scout.com. He considered Arizona, North Carolina, Wake Forest and Georgia Tech before finally committing to the University of Kansas on May 21, 2004.
Chalmers began his college career on the bench behind guard Jeff Hawkins, but worked his way into the starting rotation in January 2006. He finished the season starting 21 of the final 22 games (missing only senior night). He finished his freshman year averaging 11.5 points per game, second on the team. He became the second freshman in Big 12 history to win Conference Player of the Week honors and was named National Freshman of the Week twice. He also set the KU and Big 12 record for most steals in a season by a freshman with 89. Chalmers won KU's Clyde Lovellette Most Improved Player Award (co- with Julian Wright), and the Dutch Lonborg Free Throw Percentage Award (78.8%).
After working out for 13 teams, Chalmers was drafted with the 34th overall pick in the second round of the 2008 NBA draft by the Minnesota Timberwolves. When asked immediately after being drafted how many times he had watched his overtime-forcing three-pointer against Memphis in the NCAA championship, Chalmers responded, "a million times." The rights to Chalmers were later included in a trade with the Miami Heat.
On September 3, 2008, Chalmers and fellow former Kansas Jayhawks teammate Darrell Arthur were excused from the NBA's rookie transition program for a marijuana-related incident. Police responding to their hotel room following a fire alarm at 2:00 a.m. claimed that the room smelled strongly of marijuana but none was found and no charges were filed. Fellow NBA rookie Michael Beasley was also reported to have been in the room at the time, but was not asked to leave camp. Beasley was later fined $50,000 for his involvement in the incident. Chalmers denied any connection with marijuana and was later fined $20,000 by the league for missing the rookie camp, but was not fined or suspended for any drug-related violations.
Chalmers started all 82 regular season games his rookie year and finished the season averaging 10.0 points, 4.9 assists, 2.8 rebounds, and 1.95 steals on 32.0 minutes per game. His 1.95 steals per game ranked number one among all rookies and the fourth highest among all players.
On November 5, 2008, Chalmers set a new Miami Heat team record for steals with 9 in just his fourth game in the NBA.
Chalmers had played in every game up to missing January 31 game against Milwaukee Bucks. It was the first game he had missed in his two year career. On January 31, 2010, an MRI confirmed that Chalmers has a torn ligament in his thumb. Chalmers returned from the injury a few weeks later, and played out the remainder of the season and the playoffs.
On July 8, 2010, when LeBron James announced he would be joining the Miami Heat, Chalmers changed to his collegiate number 15, and James took Chalmers' old number 6.
In Game 5 of the 2011 NBA Finals, Chalmers made a half-court shot at the buzzer of the first quarter to give the Miami Heat a 31–30 lead, but the Heat would lose the game to the Dallas Mavericks 112–103 and trail in the series two games to three. Chalmers had 15 points, four rebounds, two assists, and a steal. In Game 6, the Heat fell to the Mavericks.
On June 15, 2011, the Heat extended a qualifying offer to Chalmers, making him a restricted free agent in the offseason. On December 9, 2011, Chalmers officially re-signed with the Heat on a 3 year, $12 million deal. He recorded 29 points in a triple-overtime victory over the Atlanta Hawks on January 5, 2012.
On February 15, 2012, the NBA announced that Chalmers, along with fellow Heat player James Jones, were going to participate in the NBA All-Star Weekend Three-Point Shootout. However, Chalmers was eliminated in the first round.
Statistically, Chalmers had his best regular season since his rookie year, and had career highs in both three-point percentage and field goal percentage.
In Game 4 of the second round playoff series against the Indiana Pacers, Chalmers scored 25 points. In Game 5, he recorded 11 rebounds with 8 points.
Chalmers was highly praised for his 25 point performance in Game 4 of the 2012 NBA Finals. He scored 12 points in the fourth quarter to help the Heat win the game.
In Game 5 of the NBA Finals, Chalmers recorded 10 points, 7 assists and 2 rebounds in 34 minutes of play as the Heat and Chalmers won the 2012 NBA championship. It was Chalmers' first NBA title.
On January 12, 2013, Chalmers scored a career-high 34 points against the Sacramento Kings, making 10 three-pointers. He tied the Heat's record for most three-pointers in a game. On June 9, 2013, during Game 2 of the NBA Finals against the San Antonio Spurs, Chalmers led all scorers with 19 points as the Heat tied the series 1-1 with a 103-84 victory. He won his second championship on June 20, 2013, as the Heat defeated the Spurs to conclude an intense seven game series.