[15][16][17][18] As a junior, Heo was a major contributor to Yonsei winning the MBC Cup for the first time in eleven years, scoring points in the double digits in three out of five tournament games.

[19] Despite having the opportunity to declare eligibility early for the 2016 rookie draft, he chose not to as he felt unprepared to go professional and to honor his parents' wishes that he complete his studies.

[28][29] Despite being evaluated as one of the more "pro ready" prospects of the draft, there were initial concerns about his height – at 180 cm he was considered short for a guard even by KBL standards.

[30][13] He made his debut in the Korean Basketball League on 7 November 2017 against Seoul SK Knights, with 15 points, 7 assists and 2 rebounds recorded in 23 minutes.

[33] Alongside fellow 2017 draftee forward Yang Hong-seok, Heo was a major contributor to KT qualifying for the play-offs for the first time in five seasons.

[35] He contributed a career-high 25 points, 17 of which was scored in the third quarter alone, in the February game against Seoul SK Knights to end KT's four-game losing streak.

[13] During the October game against Wonju DB Promy at home, he scored nine consecutive three-point shots, tying Cho Sung-won's fifteen-year-old record for the most consecutive three-point shots in a single game; KT Sonicboom had been trailing the visitors before Heo's fourth quarter record-breaking feat brought the score up but still lost 83–82.

[51][52][53] In December, he was nominated for and won the Male Basketball Player of the Year, voted for by league coaches and his KBL peers, at the annual Dong-a Sports Awards honoring domestic professional athletes across all disciplines.

[62][63] He notably scored during decisive moments in several of KT's narrow wins, such as a buzzer beater against Changwon LG Sakers with 2.2 seconds left,[64] the decisive equalizer against Ulsan Hyundai Mobis Phoebus during the last minute of the fourth quarter after orchestrating KT's comeback from a 22-point deficit,[65] and a tie-breaker against Daegu KOGAS Pegasus during the last 30 seconds.

[66] In January against Anyang KGC, he became the 48th player in KBL history to reach 1,000 career assists as KT ended a four-game losing streak to win 77–66.

[21] Early on, during his father's tenure as the national team coach, his selection was constantly a source of criticism and accusations of favouritism, which came to a head after South Korea failed to defend their gold medal at the 2018 Asian Games.

His and his older brother's selections were particularly criticized by the press, resulting in the youngest Heo being left on the substitute's bench for the entire tournament.

[81][82] Heo has participated in the 2022 FIBA Asia Cup qualifiers, but mostly as a reserve due to league commitments and quarantine requirements that discouraged his club from releasing him to the national team more frequently than usual.

[86][87][88] During the qualifiers, Kim Nak-hyeon replaced him as the first option point guard after he drew criticism for lackluster performances against lower-ranked teams; however, Heo played a key role in South Korea finishing at the top of their group at the tournament.

Domestic basketball observers and retired veterans had lamented the dearth of homegrown point guards able to both adapt to modern fast-paced offenses and also statistically match the levels of legends such as Lee Sang-min, Joo Hee-jung and Kim Seung-hyun, whose prime took place from the early to mid 2000s.

[99] The Heo brothers launched a jointly-run YouTube channel called Ko Sambuja (코삼부자), documenting their lives as professional athletes.

[102] On 16 May 2022, Heo enlisted for mandatory military service and was assigned to the Korea Armed Forces Athletic Corps' Sangmu basketball team.

Heo granted permission for his name to be used and was portrayed by Lee Seok-min, a former Myongji University player who went undrafted during the 2021 KBL draft.