Lee joined Valencia CF's academy in July 2011. On 15 December 2017, he was called up to Valencia CF Mestalla. He had his debut on 21 December 2017, in a match with Deportivo Aragón, being substituted in the 82nd minute. He had his first assist on 17 February 2018, against UE Llagostera, being substituted in at the 76th minute and assisting in extra time. He had his first goal on 6 May 2018, starting against CE Sabadell FC. He made his first team debut on 30 October 2018, starting and playing 83 minutes in a 2–1 win over CD Ebro in the Copa del Rey, becoming the youngest Korean footballer to debut professionally in Europe. Lee made his La Liga debut on 12 January 2019, substituting in for Denis Cheryshev in the 86th minute in a 1–1 draw with Real Valladolid during 2018–19 season. At 17 years, 10 months, and 24 days, Lee became the second youngest player in the 18–19 La Liga after Ander Barrenetxea from Real Sociedad who debuted at the age of 16 years, 11 months, and 24 days. He also became the youngest ever non-Spanish and the first Asian league debutant for Valencia, as well as the fifth Korean to play in La Liga. On 30 January 2019, Lee officially joined the Valencia first-team, receiving jersey number 16. On 17 September 2019, Lee made his UEFA Champions League debut as a late substitute for Rodrigo in a 1–0 win against Chelsea, becoming the youngest ever Korean to debut in the competition at the age of 18 years and 6 months, a record previously held by Jeong Woo-yeong. He also became the 5th youngest ever to debut for Los Ches in the same competition.
Lee was selected to play for the South Korea U-20 football team in the 2019 FIFA U-20 World Cup and played in every group and knockout match, leading his team to a historic runner-up finish in the tournament and scoring two goals and four assists in seven games. As a result, he received the Golden Ball award as the tournament's best player.
In 2007, the six-year-old Lee was featured in the third season of the KBS N Sports reality football show Shoot-dori. After his exceptional performance, he passed through Yoo Sang-chul's youth academy and in 2009 joined the Incheon United FC U-12 youth team. Lee later attended Seokjeong Elementary School in Incheon and played for Flyings FC. In January 2011, he went to Spain under the recommendation of his youth team coach and underwent tests for the Villarreal CF and Valencia CF youth teams.
Personal life
He was born on 19 February 2001 in Incheon, the youngest of three children. His father Lee Woon-seong was a taekwondo instructor in Incheon and a big football fan.
