KT Sonicboom (2017–2025) 2017–2020 Heo was drafted by
Busan KT Sonicboom in the
2017 rookie draft as their first pick in the first round. 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. His rookie season was overshadowed by an injury which sidelined him for almost a month, him being forced to play in an unfamiliar hybrid shooting guard-cum-point guard role, and KT finishing at the bottom of the league table, but he managed to average 10.6 points in 26 games and establish himself as a regular by the end of the season. During Heo's second season, new head coach Seo Dong-chul switched tactics and effectively paired him with another guard to alternate between the shooting guard and point guard roles. 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. The 2019–20 season was Heo's break-out season, despite the disruption caused by the
COVID-19 pandemic during the last few rounds. In February 2020 against
Anyang KGC, he put up 24 points and 21 assists, becoming the first player in KBL history to score over 20 points and 20 assists in a single game. In that same game, he also reached the KBL record for the most assists in a quarter, with eight, tying with
Kim Seung-hyun and
Moon Tae-jong. Among domestic players, he was the top-scoring guard and ranked second only behind forward
Song Kyo-chang of
Jeonju KCC Egis by a slim margin of 0.1 points, earning him a reputation as a
combo guard equally proficient at scoring points and providing assists. Despite his performances and productive partnership with Yang, KT was only able to finish the season in sixth place, the final play-off spot. He was named into the
KBL Best 5 for the first time and voted
regular season MVP for the 2019–20 season. With this win, Heo and his father became the first father-son duo in domestic basketball history to win an MVP award; his father had previously won the equivalent several times during the semi-professional pre-KBL era and a play-off MVP in the first season of the newly established KBL. Additionally, his nine consecutive three-pointers were voted "Play of the Season".
2020–2022 The 2020–21 season saw Heo achieve a number of milestones and records. He ended the season with an average of 15.63 points per game, surpassing his previous personal record, and led the league in assists overall (among both domestic and foreign players) for a second consecutive season. Additionally, he became the first domestic player to lead in points scored (among domestic players) and assists (overall), as well as the first domestic player since
Seo Jang-hoon to lead in both the
domestic and overall statistical categories; Seo led in points scored (domestic) and rebounds (overall) for the 1998–99 season. 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. During the March game against Anyang KGC, he and Yang both recorded
double-doubles, marking the first time in KBL history two domestic players recorded double-doubles (across all categories) in a single game. Heo's double-double was his tenth of the season, the first time since the 2006–07 season a player has achieved ten or more double-doubles in points and assists. He was also named Player of the Year (KBL category) and voted into the "Best 5" of
Rookie the Basket magazine's annual Players' Choice Awards. Heo ruptured his ankle ligaments during a pre-season practice game in September and was out for the entire first round of the 2021–22 season. He returned for the November game against
Changwon LG Sakers and contributed 20 points in a 89–80 win. The aftereffects of his injury continued to affect him for the next two months, limiting his playing time. Although his statistics were lower than the previous two seasons, he contributed by scoring in
clutch situations and during the second half. 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, 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, and a tie-breaker against
Daegu KOGAS Pegasus during the last 30 seconds. 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. In March, he achieved his first double-double (points and assists) of the season.
2022–2025 Heo enlisted for mandatory military service and was assigned to the Sangmu basketball team after the 2021–22 season ended. His contemporaries included several of his national teammates and they led the team to win the first D League title in four years. He scored 26 points in the final against
Changwon LG Sakers. Despite not participating in pre-season training, he immediately returned to the line-up just days after his discharge and scored 26 points against Seoul SK Knights in his first game of the 2023–24 season. However he struggled with injuries, including a broken nose in December, and ankle and calf problems that ruled him out for much of January In spite of his injury struggles he maintained an average of 14.9 points in the 13 games he had played in before the January injury lay-off.
Busan KCC Egis eventually won the finals 4-1 but Heo was singled out for praise, with various media publications summing up KT's performance during the series as Heo's "one-man show" and KCC coach Jeon Chang-jin praising him in the post-celebrations interview. He put up 37 points in Game 3, the second highest ever recorded by a domestic player during a Championship game, behind his father's former Busan Kia Enterprise teammate
small forward ; he shares the record with Cho Sung-won and Kim Sun-hyung. Ahead of the 2024–25 season, Heo took over
Moon Seong-gon as team captain. His form was inconsistent during the early half of the season due to various minor injuries and overexertion. Due to the lack of consistent scoring options, he was forced to play the first eight games of the season while nursing an injury to his non-dominant (left) wrist, averaging 17.4 points and recording double-doubles in two of those games. The KBL's newly implemented Asian quota allowed KT to sign Filipino rookie guard
JD Cagulangan in January, which freed him to return to his playmaking role. He regained his form by Round 6 as KT was battling for a place in the playoffs, ultimately finishing fourth for a second consecutive time in the regular season.
KCC Egis (2025–present) Heo's contract with KT ended after the 2024–25 season and was widely expected to re-sign. In May,
Busan KCC Egis announced that he had signed a five-year contract, making him the highest-paid domestic player in the league along with veteran
Kim Sun-hyung. ==National team career==