In 2002, Lee successfully signed a contract with S.M. Entertainment after she won the category of "Best Feature" in the SM Entertainment Best Youth contest. For the contest, she acted out a monologue, singing to Dana The Grace's "Until the End of the World", and also did freestyle modeling poses. After she joined the company, she began intense training lessons on acting, singing, and dancing.
Soon after she joined SM Entertainment, Lee began her career by starring in music videos for SM artists, such as boy band TVXQ, Moon Heejun, KangTa, Shinhwa, and Fly to the Sky, overtaking SM artists Kim Bomi (of M.I.L.K.) and Lee Jiyeon (commonly known as LinA, of CSJH The Grace) as "The SM music video girl". In the span of one year, she starred in eight music videos compared to Kim Bomi's and Lee Jiyeon's five each within two years. Though Lee had relatively steady work between modelling in magazines, commercial ads and appearing in music videos, she did not officially debut until late 2004. In November, she had undertaken a role in the popular historical drama Emperor of the Sea, and was featured a few months later in the latter episodes of the daily drama My Lovely Family. Her subsequent role in Resurrection brought the number of her drama appearances to three. Lee kept busy throughout the rest of 2005 by filming many commercials and modelling for various magazines. Lee was also in the first line-up of Girls' Generation during the creation process of the group. After being cut from the group, she decided to continue her career as an actress.
Lee's big break came in the form of the movie A Millionaire's First Love where she played a lead role alongside popular actor Hyun Bin. Lee resolved to "act as hard as she can with all that she's learned" for her movie debut. Through the movie she also made her singing debut, singing "Do-Re-Mi" from The Sound of Music and Insa on the film's original soundtrack.
Lee's name as "music video girl" became more apparent after labelmate Zhang Liyin's 2006 "Timeless" music video, where she starred alongside Super Junior's Han Geng and Choi Siwon. Two years later, the trio reunited for Zhang's "I WILL" and "The Left Shore of Happiness", which were released in early 2008 in China and weeks later, Korea. Together, netizens commonly termed the trio as "The Timeless Cast". She once again reprised her title in 2010, playing an angel in the label's project group S.M. The Ballad's music video for their debut single "Miss You".
In 2009, she starred as one of the main cast in East of Eden. Unfortunately her acting in the series was poorly received, with many calling it "weak" and "not living up to her full potential". As a singer, she was featured on Super Junior's third album Sorry Sorry.
In 2011, as a celebration for her 23rd birthday, the actress held a photo exhibition/fan meeting. The exhibition went by the name of 'Time' and was held from February 11 to 13. During the exhibit, the star received much attention from the press as well as special visits from friends and labelmates. In 2012 she was listed as one of the Top 6 most beautiful female celebrities of SM Entertainment.