Sung turned to acting in 2002, starring in television dramas such as Thousand Years of Love (2003), The Snow Queen (2006), Hong Gil-dong (2008), and Feast of the Gods (2010).

[3] This was followed by the romantic comedy First Love of a Royal Prince (2004), partly shot in Japan and Bali, in which Sung's fun-loving sandwich shop delivery girl was a departure from her previous feminine characters.

[5][6][7] Sung graduated from Kyung Hee University in February 2005 with a degree in Theater and Film; she also received an Achievement Award for promoting her alma mater.

In 2006, she starred in One Fine Day and The Snow Queen, playing, respectively, an orphan adopted by a wealthy but suffocating family, and a cold-hearted heiress with an incurable disease.

The series' screenwriters, the Hong sisters were initially criticized for casting her, but they defended Sung, and her portrayal of a tomboyish, vanity-free outlaw drew a positive reception from audiences.

[9] Sung then played an aspiring show director who joins the Cirque du Soleil in Swallow the Sun, a 2009 big-budget action-romance series with overseas location shoots in Las Vegas and South Africa.

[10] That same year, she also appeared in her first big-screen starring role as a Korean-American adoptee who returns to her native country to search for her biological mother in Maybe (titled Rabbit and Lizard in Korean).

[7][20][21][22][23] Afterwards, Sung reunited onscreen with previous Hong Gil-dong co-star Kang Ji-hwan in the action-comedy film Runway Cop, which revolves around a detective who goes undercover as a fashion model for a drug case.