After being named to the all-freshman team in the Atlantic Coast Conference (ACC), he was selected in the first round of the 2018 NBA draft by the San Antonio Spurs with the 18th overall pick.

On March 25, 2017, Walker led Reading Senior High School to their first state championship in the program's 117-year history in a 64–60 win over the Pine-Richland Rams.

[4] During the season, Walker signed with the Miami Hurricanes, passing on other scholarship offers from Villanova, Kentucky, Syracuse, and Arizona.

[1][5] Sources: Walker made his collegiate debut with the Miami Hurricanes on November 10, 2017, recording 10 points and a season-high five assists coming off the bench in a 77–45 blowout win over Gardner-Webb University.

[13] Walker made his NBA debut on January 3, 2019, in a 125–107 win against the Toronto Raptors, scoring three points and grabbing one rebound in five minutes of play.

[14] On December 3, 2019, Walker scored a career-high 28 points, including 19 in the 4th quarter, in a 135–133 double overtime win over the Houston Rockets with four rebounds, three steals and a block.

[19] Walker returned after missing 14 games,[17] but his playing time was limited with the development of Austin Reaves and particularly after the Lakers acquired D'Angelo Russell, Malik Beasley, Jarred Vanderbilt, and Rui Hachimura before the trade deadline.

[27] In June 2020, Walker revealed he had been the victim of sexual abuse by family members as an adolescent, and had grown out his trademark long hairstyle as a coping mechanism.

In deciding to cut his hair, Walker described the act as having "shed my skin mentally, emotionally, physically and spiritually.