On December 18, 2018, Lee played in KeSPA Cup 2018 under the amateur team, KeG Seoul. His team defeated pro LCK team, Hanwha Life Esports with a match score of 2–1.[10][11]
In December, it was announced that Lee joined SK Telecom T1 under the name "Catan".[12]
2019
During his time as trainee, he and four other trainees from T1, Canna, Ellim, Mask, and Kuri, competed in the 2019 LoL Amateur Tournament. The team was called 'T1 Rookies', and they won the tournament.[13][14]
On November 26, 2019, T1 announced that Lee was called up to the main roster.[15]
2020
Lee made his debut in the LCK on Regional Finals.[16] His debut match was against Afreeca Freecs, which he won with a match score of 3–1.[17]
On January 13, 2021, Lee played his first game of the season in LCK Spring 2021. He served as the main player of the roster for the first time in his career. The game was against Hanwha Life Esports.[19]
Lee got his first pentakill in his career in the match against DWG KIA. He achieved the milestone with Aphelios.[20]
Lee broke the record of the player with the most kills in a season, setting the record to 219 kills.[24] He was also a member of the team that went undefeated in the regular season with a 18-0 record, becoming the first team to achieve this record in the LCK.[25]
Lee won his first LCK title with his teammates in the 2022 LCK Spring finals.[26]
2023
Lee and T1 reached LCK Spring finals, where they lost to Gen.G. With finishing second place but qualified for the Mid-Season Invitational as the second seed, where they got defeated 3–2 by JD Gaming and 3–1 by Bilibili Gaming to exit the Mid-Season Invitational at the loser-bracket final. T1 finishing second in LCK Summer Finals in another rematch against GenG, with the most championship points in the LCK, the team qualified for the 2023 World Championship, marking Lee's third Worlds appearance. At Worlds, Lee and T1 won their fourth Worlds title on November 19, 2023. After going 3-1 in the Swiss Stage, Lee and T1 entered the Knockout Stage, where they first defeated the LPL second seed LNG Esports 3-0 in the quarterfinals. After defeating tournament favorites JD Gaming 3-1 in the semifinals, Lee and T1 defeated Weibo Gaming in the finals 3-0, marking Lee's first World Championship and T1's fourth.
On November 23, 2023, Lee re-signed with T1.
2024
At Worlds, Lee and T1 won their fifth Worlds title on November 2nd, 2024.[27]