He was picked ninth overall by the Charlotte Bobcats in the 2011 NBA draft and also played for the Boston Celtics, New York Knicks, and the Dallas Mavericks, before finishing his career with AS Monaco.

In their 2010–11 season, Walker was the nation's second-leading scorer and was named consensus first-team All-American; he also led the Huskies to a 2011 NCAA championship victory and claimed the tournament's Most Outstanding Player award.

During his junior year, Walker played once at Madison Square Garden against Simeon Career Academy and senior guard Derrick Rose in a 53–51 victory.

Connecticut clinched a 4-seed in the NCAA's 2010 National Invitation Tournament only to fall short to Virginia Tech and finish the relatively disappointing season going 18–16 (7–11) in conference play.

This was because, twelve games into his junior campaign, he distinguished himself as the leading scorer in the nation, posting 26.7 points per contest; in addition, Walker averaged 5 rebounds and 3.8 assists.

Walker led UConn to victory in the ensuing Big East Championship against the Louisville Cardinals and earned tournament MVP honors in the process.

[11] His heroic performance in the 2010–11 season also made him a finalist for the Association's college player of the year honors, the Oscar Robertson Trophy.

[12] Among other accolades bestowed on him, Walker claimed the Bob Cousy Award for college basketball's top point guard in the nation.

[13] On April 4, 2011, Walker, with 16 points himself, rallied and led the University of Connecticut to win the Division I Championship, while he won the honor of NCAA basketball tournament Most Outstanding Player.

In a home victory over the Orlando Magic on April 4, 2014, Walker recorded his second career triple-double with 13 points, 10 assists, and 10 rebounds.

[25] In the newly renamed Charlotte Hornets' opening game against the Milwaukee Bucks on October 29, Walker led an impressive comeback in an overtime 108–106 victory.

The Hornets had been 24 points down by the fourth quarter when they staged a comeback; Walker scored a three-pointer with 1.6 seconds left to force overtime and later hit the shot that clinched the game as well.

[33] On January 28, 2015, Walker was ruled out for six weeks after he underwent successful surgery to repair a torn lateral meniscus in his left knee.

[34] He returned to action on March 11, after missing 18 games, to score six points in 16 minutes off the bench as the Hornets lost to the Sacramento Kings 106–113.

He made 16-of-34 from the field, including 6-of-11 three-pointers, and was 14-of-15 from the free throw line, breaking the team mark of 48 points set by Glen Rice in March 1997.

[37] On March 9, in a win over the New Orleans Pelicans, Walker became just the third Hornets player to make 500 career three-pointers, joining Dell Curry (929) and Glen Rice (508).

[48] Two days later, in a loss to the Sacramento Kings, Walker moved into third place in franchise history in field goals made (2,586), passing Gerald Wallace.

[53] On December 4, Walker scored 29 points and made all 14 free throw attempts (tying a career-high) in a 104–94 win over the Orlando Magic, becoming the first player in franchise history with 200 career 20-point games.

[64] Three days later, he scored 39 points, including the game-winning free throw with a half-second left, to lift the Hornets to a 113–112 win over the Miami Heat.

[66] On December 5 against the Minnesota Timberwolves, Walker made his 502nd career start for Charlotte, the most of any player in team history, passing Muggsy Bogues (501).

[73] Coming off a frustrating 2018–19 season, the Boston Celtics would go on to lose franchise centerpieces Kyrie Irving and Al Horford to free agency, opening the door to start a new chapter with a new max-salary player.

After Walker and the Celtics showed mutual interest at the start of free agency, he officially joined the franchise on July 6, 2019, through a sign and trade deal with Charlotte worth $141 million over four years.

[75] Walker made his Celtics debut on October 23, 2019, against the Philadelphia 76ers; he put up 12 points on 18 shot attempts from the field in 34 minutes of play during the 107–93 loss.

Posting 20 points, seven assists, four rebounds, and a steal, he helped the Celtics come away with a 139–107 blowout victory over longtime rivals the Lakers.

After the season resumed, Walker helped the Celtics reach the Eastern Conference Finals, where the team was defeated by the Miami Heat.

[82] In December 2020, the Celtics confirmed that Walker had received a stem cell injection in his left knee in October and had been placed on 12 week strengthening program, sidelining him for the start of the 2020–21 NBA season.

[88] On October 20, Walker made his Knicks debut, recording 10 points and eight assists in a 138–134 double overtime victory over the Boston Celtics.

[95] On July 6, 2022, Walker and the draft rights to Jalen Duren were traded to the Detroit Pistons in exchange for a future first-round selection.

[100] On July 21, 2023, Walker signed with LNB Pro A and EuroLeague club AS Monaco, moving outside of the United States for the first time in his career.

[101] On October 18, Walker made his debut for Monaco, scoring 2 points in ten minutes of play in a EuroLeague home win over Crvena zvezda.