So-young debuted in the TV dramaLove Tomorrow in 1993 and quickly established herself as a representative star of her generation. She made her film debut opposite Jung Woo-sung in The Fox with Nine Tails in 1994, which ranked as the first Korean film ever to use computer-generated images. However, it failed to make an impression on audiences or critics. Ko first achieved wide recognition through her role in Beat (again with Jung Woo-sung), a film that caught the imagination of many South Korean high school students. Since then she has acted in a series of successful melodramas, portraying a young model in If the Sun Rises in the West, a Jeju Island tour guide in Love Wind, Love Song, and a Korean American adoptee in Love.
In 2001, So-young teamed up with actor Lee Sung-jae in A Day, about a married couple who have trouble conceiving a child. Her acting in the film was much praised and garnered the Best Actress prize from the local Grand Bell Awards ceremony. Then, after two years off, Ko returned in 2003 opposite Han Suk-kyu in the spy thriller Double Agent; however, the film failed to live up to the expectations of most viewers and critics. She then shot two films in 2006: the horror film APT by director Ahn Byeong-ki, and the romantic comedy Project Makeover by debut director Jeon Young-gap.
Ko married actor Jang Dong-gun in a star-studded wedding ceremony held at Seoul's Shilla Hotel on May 2, 2010, while five months pregnant. The couple's first child, a son Jang Min-joon, was born on October 4, 2010. and expected second child  Their second child, a daughter, was born on February 25, 2014.