Kim So-yeon broke into the entertainment industry in 1994 at the age of 14 by entering the Miss Binggrae beauty pageant without her parents' approval. Then in her second year of middle school, she used a marker pen as her eyeliner and ended up winning the top prize. Not long after her acting debut in Dinosaur Teacher, the teen Actress starred in popular TV shows such as Reporting for Duty (1996) and Soonpoong Clinic (1998), while hosting the music program Inkigayo and appearing in numerous commercials (Kim was the first Korean teen star to earn more than ₩100 million from commercial modeling). Because of her preternaturally mature looks and poise, she was often cast in older roles, notably as a manipulative, villainous anchorwoman in All About Eve, which drew a peak viewership rating of 45.2% when it aired in 2000.

But being typecast as prim, cold-hearted characters had a negative effect on Kim's career, and her fame went downhill in the mid-2000s. Her attempt to crossover to China via Hong Kong Director Tsui Hark's wuxia film Seven Swords (in which she played a Joseon noblewoman-turned-slave) and the Chinese dramas Just Like a Beautiful Flying Butterfly and Anhui Merchants also proved unsuccessful. So Kim took a break for three years to ponder whether she wanted to continue being an Actress. She returned to television in 2008 with Gourmet (adapted from Huh Young-man's titular manhwa) and found acting fun for the first time.

A year later, Kim's career resurgence began with the 2009 big-budget series Iris. She cut short her trademark long hair and worked hard on her action scenes to look convincing in her role as a North Korean spy, and she drew praise for her portrayal of a tough woman conflicted about her loyalty to her country and her love for a South Korean agent. Kim later reprised the role in the re-edited cut Iris the Movie, the series' spin-off Athena: Goddess of War (2010) and sequel Iris II (2013).

Kim was cast in the leading role in romantic comedy Prosecutor Princess in 2010, and showcased her versatility by playing a Legally Blonde-inspired newbie district attorney who's initially more interested in shopping and dating, yet surprises her colleagues with her intelligence and learns to strive for justice. Kim said that the cheerful, unpredictable character was similar to her own real-life personality. Later in the year, she played a rigid Doctor in Sports drama Dr. Champ.

In 2012, Kim appeared on the big screen as a royal barista embroiled in a plot to assassinate Emperor Gojong in Gabi, adapted from the historical fiction novel by Kim Takhwan. She spent months studying the Russian language and coffee brewing even before shooting began. Gabi was Kim's first Korean feature film in 15 years, since Change in 1997. This was followed by another period drama The Great Seer, where she played a healer in Goryeo.

Kim reunited with Prosecutor Princess Screenwriter So Hyun-kyung in Two Weeks (2013), in the role of a prosecutor who teams up with a fugitive to bring down a corrupt Politician and her gangster henchman. In 2014, she played a fashion merchandiser who falls for her much younger childhood friend in I Need Romance 3. Kim then appeared in the "Female Soldier Special" of military boot camp reality show Real Men.

In 2015, she starred in her second cable series Beating Again, about a businessman who falls for his secretary after he gets a heart transplant. Kim also joined the fourth season of We Got Married, pairing up with Kwak Si-yang.

On September 6, 2016, it was confirmed that Kim and actor Lee Sang-woo are in a relationship. The pair met on the set of MBC's weekend drama Happy Home. On March 28, 2017, it was announced that Kim and Lee were getting married in June 2017.

In March 2018, Kim left her previous agency Namoo Actors and signed an exclusive contract with J Wide Company on April 3, 2018. She was then cast in SBS weekend drama Secret Mother.