Harris played his high school basketball at Chelan High and was coached by his father. His junior year, he averaged 24.7 points, 8.5 rebounds, 4.3 assists and 2.7 steals per game. He was named Class 1A Player of the Year. 

During his senior season, Harris led his team to fifth place in the Class 1A state tournament after finishing with a 26-2 record. He averaged 26.6 points, 8.0 rebounds, 4.6 assists, 4.4 steals and 1.6 blocks per game. He was named Washington Mr. Basketball and the AP Class 1A Player of the Year (second time). He finished his high school career with 452 assists, 699 rebounds, 282 steals and a Class 1A state record 2,399 points.

Considered a three-star recruit out of high school, Harris was recruited by Tony Bennett when he was the coach at Washington State. When Bennett went to Virginia, Harris switched commitment to join Bennet at Virginia. 

Harris didn’t play much during his freshman season but began to become a big part of the Cavaliers plans as his college career progressed. Entering his senior season, Harris was on the preseason Wooden Award watch list. He led the Cavaliers to their first outright ACC regular-season title since 1981, their first tournament title since 1976 and their first Sweet Sixteen appearance since 1995.

Harris was drafted by the Cleveland Cavaliers with the 33rd overall pick of the 2014 NBA Draft. He was part of the Cavaliers team that made the NBA Finals during his rookie season. On January 5, 2016, Harris underwent season-ending surgery to remove an extra bone in his right foot. A week later, he was traded to the Orlando Magic. He was immediately waived by the Magic.

On July 19th, 2016, Harris signed with the Brooklyn Nets. He injured his shoulder in April of 2017 and was ruled out for the remainder of the season. He re-signed with the Nets in July of 2018. He won the NBA 3-Point Contest in February of 2019.