Lim Hyo-jun started competing as a swimmer, but later switched to short track speed skating after suffering damage to his tympanum. During his early career during middle school, he underwent seven surgeries after multiple injuries sustained during competitions caused numerous hiatuses, including a period in which Lim did not compete for a year, following a leg injury. During his early career, Lim represented South Korea in his first international competition at the 2012 Winter Youth Olympics in Innsbruck. There, he won the gold medal on his debut in the Boys' 1000 m event, and the silver medal behind fellow South Korean Yoon Su-min in the Boys' 500 m event. In the unique Mixed team relay event, where athletes of different countries and sexes competed together, Lim's "Team H" reached the gold medal final, but ultimately did not place after they were penalised for dropping the baton during the race.
After a four-year absence from international competition, Lim returned during the 2017 Winter Universiade in Almaty, once again representing South Korea. He competed in the Men's 500 m event, where he reached the final following respective first and second-place finishes in the heats and quarterfinals, and the disqualification of Kazakhstan's Nurbergen Zhumagaziyev for obstructing Lim in the semifinal. In the final, however, Lim failed to make a podium finish, placing fourth behind South Korea's Kim Do-kyoum and Kazakhstan's Abzal Azhgaliyev and Denis Nikisha. In the Men's 5000 m relay event, the South Korean team, which initially didn't include Lim due to his occupation with the 500 m event the same day of the semifinal, were successful in qualifying for the final. Lim was chosen to replace Lee Mun-hyeon for the final race the next day, though they were ultimately unsuccessful, conceding a penalty in a performance that would've otherwise saw them place third. Prior to his debut at the 2018 Winter Olympics, Lim won the 1000 m and 1500 m events at the September 2017 Budapest leg of the 2017–18 ISU Short Track Speed Skating World Cup, where he placed ahead of fellow South Korean skater Hwang Dae-heon both times. In the same leg, Lim also placed second in the 500m event ahead of Hwang and behind Hungary's Sándor Liu Shaolin. Lim continued on to the Seoul leg of the World Cup in November, where he was relatively less successful, reaching only the placement final in the 500 m event, and being disqualified in the 1000 m event following a collision with Russia's Alexander Shulginov in the quarterfinal. Lim reached the gold medal final of the 1500 m event, but was also similarly disqualified. He solely found success during the Seoul leg through the South Korean 5000 m relay team, alongside Seo Yi-ra, Kwak Yoon-gy, and Kim Do-kyoum, placing second in their heat and semifinal, before winning the final race. At the 2018 Winter Olympics, Lim Hyo-jun competed in the Men's 1500 m event, and is due to compete as part of the Korean team for the Men's 5000 m relay. In the heats of the individual 1500 m event, Lim placed first in a race that saw Japan's Kazuki Yoshinaga disqualified after a stumble; Hungary's Shaoang Liu, who placed fourth, qualified with Lim to the semifinals. In the semifinals, Lim again placed first along with fellow Korean Hwang Dae-heon, who placed second behind him. In the gold medal final, Canada's Charles Hamelin collided with Hwang with three laps left in the race, leaving Lim the last Korean in the field. He nevertheless finished the race seven hundredths of a second ahead of the Netherlands' Sjinkie Knegt, the World record holder, to claim the gold medal in the event; the third time in the event's history that a South Korean had placed first. Lim also claimed the Olympic record for the event, recording a time of 2:10.485 in the final, beating Lee Jung-su's record of 2:10.949 set at the 2010 Winter Olympics in Vancouver.
