In his two-year career at Kentucky, Harrison averaged 10.1 points, 2.7 rebounds and 3.8 assists in 79 games. Harrison and his brother, Aaron, helped lead Kentucky to successive Final Fours in 2014 and 2015, but came up empty handed both times after losing in the 2014 title game to Connecticut and 2015 the semi-finals to Wisconsin. On April 9, 2015, Harrison declared for the NBA draft, forgoing his final two years of college eligibility. He was joined alongside his twin brother Aaron and fellow Kentucky teammates Karl-Anthony Towns, Willie Cauley-Stein, Trey Lyles, Devin Booker, and Dakari Johnson.
On June 25, 2015, Harrison was selected with the 44th overall pick by the Phoenix Suns in the 2015 NBA draft. His rights were then traded to the Memphis Grizzlies in exchange for Jon Leuer. He joined the Grizzlies for the 2015 NBA Summer League and averaged 5.4 points, 2.0 rebounds and 3.0 assists in five games. On October 31, 2015, he was acquired by the Iowa Energy of the NBA Development League, the affiliate team of the Grizzlies. He made his professional debut for the Energy on November 14 in a 98–95 win over the Sioux Falls Skyforce, recording 11 points and seven assists in 29 minutes. On February 16, 2016, he scored a season-high 36 points in a 115–105 loss to the Canton Charge. In 46 games for Iowa in 2015–16, he averaged 18.5 points, 4.3 rebounds, 4.9 assists and 1.5 steals per game.
Memphis Grizzlies (2016–2018)
After re-joining the Memphis Grizzlies for the 2016 NBA Summer League, Harrison signed a multi-year deal with the team on July 12, 2016. On November 30, 2016, he scored a career-high 21 points in a 120–105 loss to the Toronto Raptors. Harrison made his debut in NBA playoffs on April 15, 2017, scoring 10 points in just under 20 minutes off the bench in a 111–82 loss to the San Antonio Spurs. On February 7, 2018, Harrison scored a career-high 23 points in a 92–88 loss to the Utah Jazz. A week later, he set a new career high with 28 points in a 121–114 loss to the Oklahoma City Thunder. Harrison missed nine games in March 2018 due to a right wrist injury. On November 1, 2018, Harrison was waived by the Grizzlies.
On September 5, 2019, Harrison signed with the Golden State Warriors. On October 19, 2019, the Warriors released Harrison. Harrison ultimately landed on the roster of the Warriors’ G League affiliate, the Santa Cruz Warriors. Harrison averaged 15.5 points, 5.1 assists, 3.8 rebounds and 1.2 steals per game.
