[2] He subsequently joined the Adelaide 36ers of the National Basketball League (NBL) as a training player for the 2018–19 season.

[3][4] In 2019, Harris helped the Eagles reach the Premier League grand final, where they defeated the Mount Gambier Pioneers 107–90 to win the championship.

[2] He was subsequently named the recipient of the Frank Angove Medal as the Premier League's Under 21 Player of the Year.

[10] He was subsequently named the recipient of the Frank Angove Medal once again, this time as the NBL1 Central's Under 23 Player of the Year.

[12][13] He helped the Pioneers reach the grand final, where they lost 78–62 to the Hobart Chargers with Harris scoring 13 points.

[17][18] He impressed coach John Rillie in his first season and soon moved up the playing rotation as he overtook fully rostered teammates,[19] coming off the bench as an impact player to shoot at 42 per cent from three-point range.

[27][28] He received a role change for the 2023–24 NBL season, becoming one of coach John Rillie's go-to defenders.