Wagner appeared in 45 games for the Wizards during the 2019-20 season. He set new per-game career highs in points (8.7), rebounds (4.9), assists (1.2), steals (0.6), threes (0.6) and blocks (0.4). He also set new career highs in all three of field-goal percentage (54.5), three-point percentage (31.3) and free-throw percentage (82.1). Wagner also set new single-game career highs in points (30), rebounds (15), assists (five), threes (four), steals (two) and blocks (three). Wagner had two double-doubles. The first was Nov. 15 against the Timberwolves, where he had 30 points, 15 rebounds, two assists, one steal and one block. The second was Nov. 24 against the Kings, where Wagner had 12 points, 11 rebounds and four assists. Wagner had two other games with 10-plus rebounds. He had 18 performances with at least 10 points, 15 performances with multiple assists, 22 performances with at least one steal and 12 performances with at least one block. He also appeared in all eight games of the Orlando bubble, averaging 5.1 points, 3.4 rebounds and 0.9 assists.

The 25th overall pick in the 2018 NBA Draft after three seasons at Michigan, Wagner appeared in 43 games (five starts) for the Lakers as a rookie. He saw 10.4 minutes per game and averaged 4.8 points and 2.0 rebounds. He also shot 41.5 percent from the field, 28.6 percent from three and 81.1 percent from the free-throw line. In his five starts, he averaged 11.4 points, 4.6 rebounds, 1.4 assists and totaled a combined eight blocks-plus-steals. Wagner's career highs were set at, respectively, 22 points, seven rebounds, three assists, two blocks, two threes and one steal. His best performance was in a 120-107 loss to the Celtics. In that game, he recorded 22 points on 13 shots, six rebounds, three assists, one steal and one block in 34 minutes. Wagner saw a significant uptick in workload when it became clear the Lakers were out of playoff contention. That fueled averages of 8.7 points and 3.1 rebounds in 18.1 minutes during his final nine appearances. Wagner was part of a successful three-man grouping, also containing Alex Caruso and Jonathan Williams, that was plus-23.9 points per 100 possessions in a 98-minute sample.