[5] Carlson played all 21 games for St. Joseph, registering 12 goals and 19 points, to help lead the team to the New Jersey State Interscholastic Athletic Association Sweet 16 tournament.

[3] While playing for the Rockets under coach Bob Thornton during the 2006–07 season,[4] Carlson scored 50 points in 44 games to earn the Atlantic Junior Hockey League Offensive Defenseman of the Year Award.

[5] The attention eventually resulted in him transferring to Cathedral High School in Indianapolis for his senior year to play one season with the Indiana Ice in the USHL.

[8] Following the NHL draft, Carlson participated in the Washington Capitals training camp and played in two preseason games before joining the Knights for the 2008–09 season.

[11] Carlson's defensive play helped the Knights hold a 9-1-0-0 record through December and he began the 2009 calendar year ranked third among all rookies in scoring.

[20] Carlson finished the playoffs with three points over 16 games en route to the Hershey Bears' 10th Calder Cup championship title in franchise history.

[28][37] While both the Bears and Capitals qualified for the postseason, Carlson remained at the NHL level for the entirety of their Eastern Conference quarter-finals series against the Canadiens.

[40] Carlson recorded one assist and six shots on net in Game 4 to help the Bears advance to the Eastern Conference Finals.

[29] Due to their play in the 2011 Stanley Cup playoffs, Boudreau reunited Carlson with his former defensive partner, Karl Alzner, for the majority of the season.

[47] As the season progressed, the duo began to earn more on-ice responsibilities such as playing against the opposing team's top players[48] and spending time on the Capitals powerplay and penalty kill unit.

[49] Despite suffering a hip injury in Game 1 of the 2011 Stanley Cup playoffs, Carlson still averaged over 25 minutes of ice time through their first round series against the New York Rangers.

[52] After the Capitals were eliminated from the 2012 playoffs, interim head coach Dale Hunter voluntarily stepped down and was replaced by Adam Oates.

[56] While Carlson was originally reunited with Alzner to start the shortened season, Oates split the duo up after four consecutive poor performances.

[66] Shortly before leaving for Sochi, Carlson set a new franchise record for defensemen by playing in his 272nd consecutive game with the Capitals.

[76] After ending the regular season with a career-high 12 goals and 43 assists, Carlson finished in 10th place in James Norris Memorial Trophy voting.

[84][85] Despite the injury, Carlson scored eight goals and 31 points by the end of the regular season to help the Capitals qualify for the 2016 Stanley Cup playoffs.

[87] In the Eastern Conference second round, Carlson tied Scott Stevens and Kevin Hatcher's franchise record for most goals by a defenseman in a single postseason.

[90] As Alzner was finding success with Matt Niskanen, the Capitals coaching staff reunited Carlson with Dmitry Orlov.

[citation needed] Carlson's 2017-18 campaign ended with a Stanley Cup victory and a fifth-place finish in Norris Trophy voting.

This goal brought his points total to 43 and tied him with Denis Potvin and Al MacInnis for the second most in NHL history by a defenseman through 30 games.

[119] On February 23, Carlson tallied an assist on Tom Wilson's goal to pass Johansson once again and become the Capitals franchise leader in points by a defenseman.

[123] When the NHL returned to play for the 2020 Stanley Cup playoffs, the Capitals moved into the Toronto bubble along with 11 other Eastern Conference teams.

[124] Upon entering the Bubble, the Capitals played an exhibition game against the Carolina Hurricanes[125] before the round-robin tournament began to determine the final playoff seeding.

[132] The team also made numerous high profile offseason acquisitions, including signing veteran defensemen Zdeno Chára.

[138] A cracked kneecap forced Carlson to miss four of the final seven games of the regular season and severely impacted his play during the 2021 Stanley Cup playoffs.

[149] While the Capitals were eliminated in the first-round 2022 Stanley Cup playoffs, Carlson extended his franchise-leads in games played, assists, and points.

[155] In the Capitals final game before the league's mandated holiday break, Carlson suffered a fractured skull and lacerated temporal artery after being accidentally struck in the head by a puck.

[163] On November 15, 2024, Carlson passed Ryan Suter and Reed Larson for sole possession of eighth place on the all-time points list for American-born defenseman.

On January 5, 2010, he scored the game-winning goal in overtime of the gold medal game as part of a two-goal effort to defeat Canada 6–5.

[23] While Team USA entered the semi-finals undefeated, they finished in fourth place after being shutout in the bronze medal game by Finland.