In July 2019, Lee Hae-in participated in the ISU Junior Grand Prix Korean qualification competition held in Taeneung, South Korea, where she came in first in the short, free, and overall program.

With two first-place finishes, Lee qualified for the 2019-20 Junior Grand Prix Final as the third-ranked skater, behind Kamila Valieva of Russia and Alysa Liu of the United States.

[4] Competing at the Junior Grand Prix Final held in Turin, Lee ranked sixth in the short program and rose to fourth in the free skate, finishing fifth overall.

At the 2020 World Junior Championships in Tallinn, Lee earned a small silver medal for the short program behind Kamila Valieva, who took gold, and ahead of Daria Usacheva, who claimed bronze.

In the free skate, she fell on a downgraded triple flip attempt, resulting in a sixth-place finish in that segment and ranking fifth overall.

She was assigned to one of Korea's two ladies' berths at the 2021 World Championships in Stockholm due to silver medalist Yun Ah-sun being age-ineligible for international senior competition.

She said that she was "super happy" with the outcome, noting that the event took place in the same arena as the 2020 World Junior Championships, where she had missed the podium due to free skate errors.

[19] In the free skate, Lee executed seven clean triple jumps, vaulting into the lead in the segment and winning the gold medal.

[20][21] Competing at the 2023 World Championships in Saitama, Lee finished second in the short program with a score of 73.62, more than five points back of segment leader Kaori Sakamoto and narrowly ahead of several other skaters.

[26] Team South Korea won the silver medal overall, with Lee and Cha Jun-hwan being their country's biggest contributors.

[29] At the 2023 NHK Trophy she replicated her earlier result with a third-place finish in the short program and then dropping to fourth due to free skate errors.

[4] Assigned next to the 2024 Four Continents Championships, she entered as the defending champion, but revealed that an incident had occurred just prior to the event that came as a "huge shock."

"[31] At the 2024 World Championships in Montreal, Lee skated a clean short program and came third in the segment, earning a bronze small medal.

"[32] The free skate proved more difficult, receiving several underrotation calls on her triple jumps, coming twelfth in the segment and dropping to sixth overall.

[33] In June 2024, Lee received a three-year suspension from the South Korean national team for both drinking alcohol with an adult female teammate, who had also received a one-year suspension, and for allegedly sexually harassing a minor male teammate during an overseas training camp held in Varese, Italy, from May 15–28, 2024.

She further stated that she and the male had agreed to rekindle their relationship in secret while at the training camp, making their text message exchanges public to back up these claims.

[35][36][37] Meanwhile, the young male's legal representative claimed that he had begun undergoing psychiatric treatment due to shock from the event that took place.

[45] This was partly due to the young male reportedly telling the Korea Skating Union during the course of their investigation that he did not believe that Lee's actions constituted as criminal behavior, and that he wished for them to show leniency and not enact a severe punishment.

[46][47] In November 12, 2024, the Seoul Eastern District Court cited a provisional injunction to suspend the disciplinary action that was issued to Lee by the Korea Skating Union.

[49] Due to being the third highest ranking senior-eligible women's skater at the event, she was ultimately named to the 2025 World team.