Two months before beginning high school, Johnson suffered open fractures in four of his fingers, was thrown about 10 feet and lost consciousness in a fireworks accident.

In his sophomore season, he averaged 25.6 points, 10.2 rebounds, and 4.1 assists per game, earning Division II-A Tennessee Mr. Basketball honors.

[3] As a junior, Johnson averaged 25.3 points, 9.4 rebounds and 3.7 assists per game, leading his team to the Division II-A state semifinals.

[14] On February 4, 2022, Johnson was traded, alongside Eric Bledsoe, Justise Winslow, and a 2025 second-round pick, to the Portland Trail Blazers in exchange for Norman Powell and Robert Covington.

[15] Johnson made his Trail Blazers debut on February 24, recording four points and four rebounds in a 132–95 loss to the Golden State Warriors.

He averaged 14.2 points per game en route to the team's second Las Vegas Summer League championship in franchise history.

[19] Two days later, Trail Blazers head coach Chauncey Billups announced that Johnson had suffered a broken finger during a practice and would be out indefinitely.