Porter was selected with the 30th overall pick in the 2019 NBA Draft after one collegiate season at USC. He was drafted by Milwaukee, but traded on draft night with Tony Snell to Detroit for Jon Leuer. Six days later, Porter was dealt to Cleveland for multiple second round draft picks. He made his NBA debut for Cleveland during the opening game of the season, on Oct. 23 against the Magic. Porter played 17 minutes and recorded two assists, one rebound and one steal. Porter had his first double-digit scoring game Nov. 8 against the Wizards when he posted 13 points (5-11 FG, 0-3 3Pt, 3-3 FT), three rebounds and a steal. He had his first 20-point scoring effort Dec. 11 against the Rockets when he delivered 24 points (9-15 FG, 6-9 3Pt), three dimes, two boards and three steals. Porter scored his career-high 30 points (9-18 FG, 4-7 3Pt, 8-10 FT) on Feb. 24 in a game against the Heat. He also posted eight rebounds, three assists and three steals in 41 minutes of play in the home win over Miami. His other career highs stood at nine rebounds, seven assists, three steals and two blocks. Porter's longest streak of double-digit scoring was seven games from Jan. 28 through Feb. 12. During that stretch, he averaged 16.1 points on 11.6 shots, 2.9 rebounds and 2.4 assists in 27.6 minutes per contest. Overall during his rookie campaign, Porter averaged 10.0 points, 3.2 rebounds and 2.2 assists in 23.2 minutes per game. Porter appeared in 50 games during the COVID19-shortened season, with three starts, and shot 33.5 percent from behind the arc.