Forbes played in all 63 pre-hiatus games for the Spurs during the 2019-20 shortened season, making 62 starts. The 6-foot-2 guard averaged a career-high 2.3 made three-pointers per game to go with 11.2 points, 2.0 rebounds and 1.7 assists per contest. Forbes came out of the gates firing on all cylinders, topping double-digit points in 14 of his first 15 contests. On Oct. 28, Forbes grabbed a season-high seven rebounds to go with 14 points during a home win over the Trail Blazers. He scored a season-high 25 points on Dec. 3 against Houston, drilling 10-of-13 shots from the field to fuel a 135-133 victory. He surpassed 20 points twice more in a six-game stretch during January, scoring 21 on Jan. 10 against the Grizzlies and dropping 24 on the Suns on Jan. 20. During the latter game, Forbes hit a career-high 8-of-11 three pointers. Forbes drained at least one three-pointer in all but six of his appearances. Before the bubble, the 26-year-old was tied with Patty Mills for the team lead in made three-pointers (148). A quad injury prevented Forbes from making any appearances in the NBA's Orlando bubble.

Forbes appeared in all 82 games for the San Antonio Spurs during the 2018-2019 campaign, starting 81 of those contests. The Michigan State product set career highs with 11.8 points, 2.9 rebounds and 2.1 assists per contest. Forbes shot 45.6 percent from the floor, including an impressive 42.6 percent from three-point range. He finished eighth in the NBA in three-point field goal percentage. Forbes notched the first double-double of his NBA career with 24 points and 11 rebounds against the Phoenix Suns on Dec. 11. The points were a season high and the rebounds set a career high. Forbes hit at least three treys in 30 contests for the Spurs. He connected on a career-best six three-pointers against Toronto on Jan. 3. Forbes averaged over 30 minutes per tilt during the Spurs' first-round playoff series against the Denver Nuggets. In seven games, Forbes averaged 10.7 points, 3.6 rebounds and 1.0 assists. He scored 19 points in Game 7 of the series.

Forbes appeared in 80 games for the San Antonio Spurs during the 2017-2018 season, his second with the franchise, and started 12 of those contests. Forbes set career highs with 6.9 points, 1.4 rebounds and 1.0 assists per contest. Forbes shot 42.1 percent from the floor, including 39.0 percent from three-point territory. He led the squad in three-point field goal percentage. He scored in double figures on 28 occasions. Forbes scored a season-high 23 points against Sacramento on Jan. 28. He hit a career-best five three-pointers in that clash. Forbes appeared in four playoff games for the Spurs. His best outing came in Game 1 of the Western Conference First Round against the Golden State Warriors. In that contest, Forbes collected 14 points and three rebounds in 25 minutes.

Forbes played for the Spurs in both the Las Vegas and Salt Lake City Summer Leagues, averaging a combined 11.6 points, 3.4 rebounds and 2.0 assists per game. His performance earned him a spot on the Summer League All-Tournament Second Team. That led San Antonio to sign Forbes to an NBA contract. He spent much of his rookie campaign with the Austin Spurs of the D-League. In 19 games with Austin, Forbes posted per game averages of 23.3 points, 3.5 rebounds, 2.7 assists and 2.9 treys. He shot an outstanding 45.9 percent from deep in the D-League. Though his playing time in the NBA was minimal, Forbes began to receive more minutes with San Antonio late in the season. On April 7, he exploded for season highs of 27 points, three rebounds, six assists and four three-pointers in a win over Dallas. That performance helped Forbes earn a spot on the playoff roster. He partook in six postseason games, averaging 12.1 minutes and 3.3 points per contest.