[2] 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.

[7] Johnson was later listed in the roster of San Antonio Spurs for 2019 NBA Summer League hosted at Vivint Arena.

On August 11, 2020, Johnson scored a season-high 24 points in a 123–105 win over the Houston Rockets with eleven rebounds and three assists.

[13][14] On October 31, 2023, Johnson put up 27 points alongside a key steal on Kevin Durant that led to a game-winner in a 115–114 win over the Phoenix Suns.

On July 16, 2021, Johnson and center JaVale McGee were named to the Olympic roster in place of the injured Kevin Love and Bradley Beal, who was kept from traveling to Tokyo for health and safety protocols.