Giles attended Wesleyan Christian Academy in High Point, North Carolina, where he was coached by Keith Gatlin.

[3] He also participated in the Under Armour Elite 24 game in Brooklyn, New York, finishing with 16 points and 11 rebounds and earning Co-MVP honors.

[5] On November 14, 2014, In his second game back since his injury, Harry scored a career-high 38 points and grabbed 19 rebounds in an 82–58 win over Northside Christian Academy.

[10] On January 15, 2015, Giles scored 17 points, 12 rebounds, and 4 assist to help the Trojans defeat Malik Monk and Bentonville High School (63–55).

[12] At the end of his junior season, Giles earned first-team All-USA honors by USA Today.

[17] Before his senior season, Giles decided to attend and play for the high school basketball powerhouse Oak Hill Academy in Mouth of Wilson, Virginia.

[18] Giles dominated the majority of his high school career; however, his senior year ended with a torn ACL in his right knee.

[20] Weeks later, Giles enrolled to (now-defunct) Forest Trail Academy in Kernersville, North Carolina to take online courses to finish his senior year of high school while rehabbing from his knee injury.

[22][23][24] He was selected to play in the 2016 Jordan Brand Classic and Nike Hoop Summit but was unable due to injury.

[39][40] On March 10, 2017, in the ACC tournament semi-finals against rival North Carolina, Giles had 4 blocks, 7 rebounds, and 6 points in a (95–83) win.

[41] On March 11, 2017, Giles contributed to 4 points and 4 rebounds in a 75–69 victory over Notre Dame in the ACC Tournament Championship game.

[45] At the conclusion of his freshman season, Giles announced that he would forgo his final three years of collegiate eligibility at Duke and enter the 2017 NBA draft.

Giles scored a season-high 19 points in Sacramento's 112–108 loss to the Oklahoma City Thunder on February 27, 2020.

[66] Giles is good friends with former Duke teammate and current NBA player Jayson Tatum.