Kim Sun-ah began her career as a model before landing supporting roles in television dramas in 1997. In 2002, she made her big screen debut in Yesterday, which won her Best New Actress at the 26th Golden Cinematography Awards. That same year, she starred in the surprise hit gross-out comedy flick Wet Dreams, in which she was nominated for Best New Actress at the 23rd Blue Dragon Film Awards. In 2004, she appeared in the romantic comedy film The Great Expectation, which earned her two Popularity trophies. In 2005, she led the hit MBC series My Name Is Kim Sam-soon, which took home the Daesang for TV at the 42nd Baeksang Arts Awards. She also received numerous acting recognitions for her performance in the series, including the Daesang at the MBC Drama Awards. In 2009, she bagged an Excellence Award at the SBS Drama Awards for City Hall. In 2011, she was paired with Lee Dong-wook in the SBS drama Scent of a Woman, which gave her a Top Excellence Award at the SBS Drama Awards. In 2018, she picked up another Daesang, this time at the SBS Drama Awards for her work in Should We Kiss First. Also in 2018, she scored a Top Excellence Award at the MBC Drama Awards for the mystery thriller Red Moon, Blue Sun.

Kim Sun-ah and her family moved to Japan when she was in middle school. She was a piano major at the Ball State University in Indiana, USA. Aside from her native Korean, she can also speak Japanese and English. In 2015, she received ₩25 million in damages after winning the lawsuit she filed against a plastic surgery clinic in Busan for using her photos and name without her permission.