2017–2018 season

In January 2018, at the 2018 senior South Korean Championships, Lee was 9th.

2018–2019 season: Junior international debut

In the 2018–2019 season, she debuted in the ISU Junior Grand Prix series. In January 2019, at the 2019 senior South Korean Championships, she won the bronze medal (behind You Young and Lim Eun-soo). In March 2019, Lee (along with You Young) represented South Korea at the 2019 World Junior Championships in Zagreb, Croatia. She ranked fourteenth in the short, which put her in only the third-to-last warm-up group for the free skate. In the free, she placed seventh, rising to eighth overall.

2019–2020 season: Second international junior season

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. Following the competition, she was selected and assigned to two ISU Junior Grand Prix events, JGP Latvia and JGP Croatia.^[4] At JGP Latvia, she became the third Korean woman ever to become an ISU Junior Grand Prix champion, following Yuna Kim and Kim Hae-jin. She finished third in the short program behind Maiia Khromykh and Daria Usacheva of Russia and placed first in the free program to win the event with a combined total of 197.63 points, more than three points ahead of silver medalist Usacheva. She set personal best scores in the short, free, and overall programs. This event marked the first Junior Grand Prix event won by a Korean lady since Kim Hae-jin's victory at JGP Slovenia in 2012.^[4] At JGP Croatia, she placed second in the short program with a new personal best, again behind Daria Usacheva. After a strong free skate in which she once again scored a new personal best, she placed first overall with a new personal best overall score of 203.40, more than six points ahead of Usacheva in second and 21 points ahead of Anna Frolova in third. This marked the first time that Lee scored above 200 points in a combined total. 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. She was the third Korean lady to ever qualify for the final after Kim Yuna and Kim Ye-lim.^[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. Lee remarked that she had not expected to be in the top three after the short program, adding, "I'm surprised with the score; it's very high."^[5] She also set a new season best for this short program. 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.^[6]

2020–2021 season: Senior international debut

With the pandemic greatly curtailing international opportunities for Korean skaters, Lee did not compete on either the Challenger or the Grand Prix circuit and instead made her competitive senior debut at the 2021 South Korean Championships. Placing second in the short program and fourth in the free skate, she won the bronze medal 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.^[7] Lee placed tenth at the World Championships, which, combined with Kim Ye-lim's eleventh place, qualified two berths for Korean ladies at the 2022 Winter Olympics in Beijing.^[8][9]

2021–2022 season: Four Continents silver

Lee made her senior Grand Prix debut at the 2021 Skate Canada International, where she placed seventh.^[10] She was tenth at the 2021 Internationaux de France, her second event.^[11]  At the 2022 South Korean Championships, the final qualification event for the South Korean Olympic team, Lee won the bronze medal, finishing 1.31 points behind silver medalist Kim Ye-lim. As a result, she was not named to one of Korea's two women's berths.^[4] All three national medalists were assigned first to compete at the 2022 Four Continents Championships in Tallinn, where Lee placed second in both segments of the competition to take the silver medal, finishing behind Japan's Mai Mihara but ahead of Kim and You Young, the two skaters assigned to the Olympic team. 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.^[12]

Lee was assigned to finish her season at the 2022 World Junior Championships in Sofia.^[12] It was later announced that she would not be competing, instead intending to compete at the Triglav Trophy.^[13] Following Kim's withdrawal from the 2022 World Championships due to a positive COVID test, Lee was named to replace her.^[14] She finished in seventh place.^[15]

2022–2023 season: World silver and Four Continents title

Lee began the season with two Challenger events, winning the bronze medal at the 2022 CS Nepela Memorial before finishing fourth at the 2022 CS Finlandia Trophy.^[4] On the Grand Prix, she finished fourth at both of her Grand Prix events, the 2022 Skate America and 2022 Grand Prix de France.^[16][17] Lee revealed after the second of these that she had been ill during the Grand Prix, impacting her stamina and performance.^[17] At the 2023 South Korean Championships, Lee won the bronze medal, earning championship assignments for the second half of the season.^[18]

At the 2023 Four Continents Championships in Colorado Springs, she was sixth in the short program after her solo triple flip received an edge call and the second half of her jump combination was called a quarter short of rotation. She said afterward, however, that her main "disappointment" was missing a level on her step sequence.^[19] In the free skate, Lee executed seven clean triple jumps, vaulting into the lead in the segment and winning the gold medal. She was the second Korean woman to win the title, after Kim Yuna in 2009.^[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. She reflected that "winning the Four Continents really helped me to get more confidence, but also put some pressure on me."^[22] Lee skated cleanly in the free skate, winning the segment with a new personal best score of 147.32. She remained second behind Sakamoto overall, winning the silver medal with a cumulative score of 220.94 and becoming the first South Korean skater to win a World medal since Kim in 2013. She called this "such a huge honor," acknowledging that "the first half of the season was rough for me. I really tried to not give up."^[23][24]

South Korea qualified for the World Team Trophy for the first time in the history of the event, and Lee joined Team South Korea in Tokyo. Lee set a personal best score of 76.90 in the short program, winning the segment over Sakamoto, who fell.^[25] Lee won the free skate as well with another new personal best (148.57).^[26] Team South Korea won the silver medal overall, with Lee and Cha Jun-hwan being their country's biggest contributors. South Korea became only the fifth country to reach the podium at the event, after the United States, Japan, Russia, and Canada.^[27]

2023–2024 season

Lee began the season at the 2023 CS Nepela Memorial, winning the silver medal. She called it a considerable improvement on her showing there the prior year, joking "I didn't fall." She said she was pleased considering she was not yet in peak physical condition.^[28] A week later she won another silver medal in an appearance at the Shanghai Trophy.^[4]

On the Grand Prix, Lee first appeared at the 2023 Grand Prix de France, where she came third in the short program after receiving quarter or underrotation calls on all of her triple jumps. After several errors in the free skate, she finished fifth in that segment and dropped to fourth overall.^[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.^[30]

Lee claimed her second Korean national silver medal at the 2024 South Korean Championships.^[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." She placed eleventh at the event, having struggled in both segments, but said "the past is the past and I will try to move on."^[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. She opined that the "audience gave me so much good energy and that was one of the reasons that I skated well."^[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. Lee reflected afterward, admitting that "this season it was really hard to believe in myself." She said she needed to work on mental strength for future years.^[33]