After being selected by San Antonio with the 29th pick of the 2019 NBA Draft, Johnson went on to spend the majority of his season in the G League with the Austin Spurs, though he also ended up making 17 NBA appearances (one start). Johnson appeared in 31 G League games, averaging 20.3 points, 5.6 rebounds and 2.4 assists over 30.2 minutes. His best G League performance came Dec. 17, when he posted 32 points, four three-pointers, eight rebounds and two assists against the Herd. On Feb. 25 with Austin, the 6-foot-5 wing generated 31 points, three rebounds, two dimes, two steals and one block against Sioux Falls. Johnson scored 20 or more points in 17 of his 31 G League appearances. Johnson rejoined the Spurs for good in March and ended up seeing a far larger role once the NBA resumed play in the Orlando bubble. On Aug. 5, he posted his first career 20-point game, shooting 7-of-10 from the field and adding six rebounds. Six days later, Aug. 11, Johnson set two new career bests with 24 points and 11 rebounds against Houston -- his first NBA double-double. He then closed out the season with another 24-point effort against Utah on Aug. 13.