Harris was born and raised in Chelan, Washington to Joseph "Joe" Harris Sr., who coached the boys' basketball team at Bridgeport High for six years and then Chelan High for 25 years, and was inducted into the Washington Interscholastic Basketball Coaches Association Hall of Fame in 2011,[6] and Alice Harris, who grew up playing softball, volleyball and basketball,[7] and is a broker and director of sales at Coldwell Banker Lake Chelan Properties.

In his junior year, he averaged 24.7 points, 8.5 rebounds, 4.3 assists and 2.7 steals a game, and led the Mountain Goats to the Class 1A state tournament with a 24–6 record.

[11] In his senior year, Harris led his team to fifth place in the Class 1A state tournament after leading them to a 26–2 record.

[12] He averaged 26.6 points, 8.0 rebounds, 4.6 assists, 4.4 steals and 1.6 blocks a game, while shooting 59.2% from the field and 76.5% from the free throw line.

[15] In Harris' collegiate debut for the Virginia Cavaliers, he recorded 2 points, 4 rebounds and 3 assists in a 76–52 win over William & Mary.

[18] In 35 minutes of playing time, Harris scored a then-career-high 24 points on 8-of-12 field goals and grabbed 5 rebounds against Minnesota at the 2010 ACC–Big Ten Challenge, winning by 87–79.

[21] On February 5, Harris played for a career-high 41 minutes and recorded 18 points, 3 rebounds, 2 assists and 3 steals in a 68–70 overtime loss against Miami.

[23] On March 1, Harris recorded an ACC career-high 19 points on 4-of-7 three-point attempts, 5 rebounds, 3 assists and 2 steals, leading the Cavaliers to a 69–58 victory against NC State.

[26] At the Cavaliers' first of three Paradise Jam tournament games, Harris scored 13 points, grabbed 6 rebounds and gave 3 assists in a 55–57 losing effort against TCU.

[31] On February 25, Harris recorded 12 points, 5 rebounds, 3 assists and a career-high 3 blocks against North Carolina in a 51–54 losing effort.

[37] He also suffered a concussion in a game against Florida State on March 1, when an opposing player, Bernard James, accidentally elbowed the side of his head.

He started out the season scoring 19 points, handing out 3 assists and grabbing a season-high 8 rebounds in a 59–63 defeat against George Mason.

[40] In the first round of the 2012 NIT Season Tip-Off tournament, Harris recorded 15 points, 3 assists and 7 rebounds in a 54–45 victory against Fairfield.

[46] On February 28, Harris scored a career-high 36 points against Mike Krzyzewski's Duke Blue Devils in a 73–68 victory.

On July 24, he signed his rookie-scale contract with the Cavaliers after averaging 7.8 points, 1.5 rebounds and 1.5 assists during the 2014 NBA Summer League.

[54] Due to LeBron James's return and Kevin Love's acquisition, Harris rarely saw minutes and was assigned to the Canton Charge of the NBA Development League (D-League) on January 20.

On February 6, he scored a game-high 23 points on 6-of-15 field goals including 4 three-pointers in a 102–84 victory over the Sioux Falls Skyforce.

[68] On October 19, 2018, in a 107–105 win over the New York Knicks, Harris' first three-pointer was his 237th as a Net, moving him into 10th place on the franchise's career list.

[69] On November 4, in a 122–97 win over the 76ers, Harris made a three-pointer in his 25th straight game, moving into sole possession of fourth place on the Nets' career list.

During a 93–106 loss to the Miami Heat on October 27, 2021, Harris became the franchise leader in career three-pointers, surpassing the 813 made by Jason Kidd for the Nets.

[84] On July 6, 2023, Harris was traded to the Detroit Pistons, alongside two second-round draft picks, in exchange for cash considerations.

[89][90] Harris wore a fluorescent green band around his left wrist in memory of his late cousin, Tricia Haerling.

[92][93] Harris is a member of the "Starting Five", along with Malcolm Brogdon, Justin Anderson, Anthony Tolliver and Garrett Temple.

[94] Harris traveled with Brogdon and Anderson to Tanzania to witness the opening of the first well they funded in July 2019, and by November, Hoops2O had raised nearly $400,000.