The 2019-20 season marked Mathews' rookie campaign. He made his NBA debut with the Wizards Oct. 23 against the Mavericks. He played one minute and racked up an assist. Overall, he appeared in 18 games, averaging 5.4 points, 1.3 rebounds, 1.1 threes, 0.6 assists, 0.4 steals and 0.1 blocks in 12.6 minutes per game. He shot 42.9 percent from the field, 41.3 percent from three and 91.2 percent from the free-throw line. On Dec. 30 against the Heat, Mathews scored a career-high 28 points. He shot 6-of-9 from the field, 4-of-7 from three and 12-of-13 from the free-throw line while also providing four rebounds in 29 minutes. That performance was the first game in Mathews' three-game streak of double-digit points. During that stretch, he averaged 18.7 points on 7.3 shots, as well as 2.3 rebounds and 0.7 assists in 23.6 minutes. Mathews' other single-game career highs were set at five threes, five steals, four rebounds, two assists and one block.