Johnson was rated as a five-star recruit and ranked as the 13th best player in the 2018
class by 247Sports He committed to University of Kentucky to play college basketball,
being joined by fellow recruits Immanuel Quickley, E. J. Montgomery, Ashton Hagans, and Tyler Herro.
As a freshman, Johnson averaged 13.5 points and 5.9 rebounds per game
shooting 46.1 percent from the field and 38.1 percent from behind the arc
He scored 19 points in a win over top-ranked Tennessee on February 16, 2019
Johnson pulled down a career-high 17 rebounds in a victory over Auburn.
Following the season he declared for the NBA draft. On June 20, 2019,
Johnson was selected with the 29th overall by the San Antonio Spurs in the 2019 NBA draft.
The Spurs obtained the draft pick used to draft Johnson in a 2018 trade; in that trade,
Spurs small forward Kawhi Leonard was sent to the Toronto Raptors in exchange for Raptors
shooting guard DeMar DeRozan, center Jakob Pöltl, and a draft choice.
Johnson was later listed in the roster of San Antonio Spurs for 2019 NBA Summer
League hosted at Vivint Arena. Johnson signed with the Spurs on July 1, 2019.
On October 30, 2019, Johnson received his first assignment to
the Austin Spurs, the Spurs’ affiliate team in the NBA G League