Johnson was selected by the San Antonio with the 29th pick of the 2019 NBA Draft. The Spurs rookie spent the majority of his time in the G League with the Austin Spurs but did appear in nine games at the NBA level for San Antonio before the hiatus. The Kentucky product made his NBA debut on Nov. 22 with a brief appearance during a loss at Philadelphia. He scored his first NBA points in his second NBA appearance on Jan. 4. In that game, he delivered two points and two rebounds in four minutes during a loss at Milwaukee. Johnson appeared in 31 G League games for the Austin Spurs and averaged 20.3 points, 5.6 rebounds and 2.4 assists over 30.2 minutes per contest. He had his best G League performance on Dec. 17 when he posted 32 points, four three-pointers, eight rebounds and two assists in a loss to the Herd. On Feb. 25 with Austin, the 6-foot-5 wing generated 31 points (13-18 FG, 2-3 3Pt, 2-2 FT), three rebounds, two dimes, two steals and one block during a loss at Sioux Falls. Johnson scored 20 or more points in 17 of his 31 G League appearances. Back with San Antonio in March, Johnson held a bigger role for the remainder of the season and into bubble play. Across his final 12 appearances from Mar. 3 to Aug. 13, Johnson averaged 11.8 points, 4.3 rebounds, 1.2 steals and 1.0 assists in 22.9 minutes. He was especially productive in the final six of eight bubble games, averaging 16.0 points, 4.7 rebounds, 1.2 assists and 1.2 steals in 25.5 minutes.