She is best known for her roles in the films Untold Scandal (2003) and Feathers in the Wind (2004), and the television series Spring Waltz (2006), Temptation of an Angel (2009) and Dong Yi (2010).

Lee So-yeon first drew notice in the period film Untold Scandal, playing the Cécile role in the 2003 Korean adaptation of Les Liaisons dangereuses.

She has since played villains in several television dramas, namely Super Rookie,[4] Spring Waltz,[5] Temptation of an Angel, and in her most high-profile yet, Korean history's famous femme fatale Jang Hui-bin in Dong Yi.

More lighthearted fare followed with My Life's Golden Age (about a family of siblings), and My Love By My Side in which she played a cheerful single mother.

Feathers in the Wind in particular brought her critical raves, with Darcy Paquet of Koreanfilm.org crediting Lee's "considerable screen presence" in making her character "memorable.