Sung Hyun-ah placed second runner-up (or third place) at the Miss Korea pageant in 1994. She was the country's representative at the 1994 Miss International pageant, where she was a semi-finalist and won Miss Photogenic.

Sung made her acting debut in Salut D'Amour, and continued to appear in TV dramas such as See and See Again (also known as Can't Take My Eyes Off You), Should My Tears Show, Hur Jun, One Day Suddenly, Bad Woman, Good Woman, Yi San, and Flames of Desire. She also released an EP in 2004 titled Turn It Up under the name A.One.

But Sung became best known for her strong, sexy roles in films such as Hong Sang-soo's Woman is the Future of Man, Kim Tae-eun's The Intimate, and Kim Ki-duk's Time. She also appeared in The Scarlet Letter and The Customer Is Always Right. In 2007 Sung won the Best Actress award from the Malaga International Fantastic Film Festival in Spain, for her role in the horror film Cello.

She married businessman Heo Eun-gyo on December 9, 2007 at the Paradise Hotel in Busan. After she and Heo divorced in 2010, Sung remarried three months later and gave birth to a son in 2012.