Kim Ah-joong first rose to fame for her role in the romance film When Romance Meets Destiny (2005). Thereafter, she appeared in several commercials and landed first leading role in daily soap opera Bizarre Bunch. However, it was her role in the highly successful comedy film 200 Pounds Beauty that pushed her to stardom. For her role as an overweight girl who undergoes cosmetic surgery makeover to become a pop sensation, she was awarded Best Actress at the 2007 Grand Bell Awards. She released soundtracks of the film, "Beautiful Girl" and the Korean version of the song "Maria". "Beautiful Girl" won Digital Music Awards' "Song of the Month" by Cyworld.

Kim returned to acting three years later following 200 Pounds Beauty, starring alongside Hwang Jung-min in romantic comedy The Accidental Couple. In 2010, Kim was cast in a U.S.-Chinese film project Amazing, a sci-fi film about the development of a virtual-reality basketball game. The same year, she was cast in medical drama Sign by Kim Eun-hee, playing a forensic scientist.

In September 2011, Kim was found guilty of tax evasion. She also had to pay an additional $600,000 fine. She and her management at the time parted ways due to the verdict. Due to this scandal she was banned from TV work in South Korea from late 2011 to late 2014, and she focused on films. In December 2012, Kim co-starred with Ji Sung in the romantic comedy film My PS Partner. This was followed by another romantic comedy film Steal My Heart in 2013.

She returned to TV in legal drama Punch, where she received positive reviews for her portrayal of a prosecutor. In 2016, Kim starred in the crime thriller Wanted. In 2017, she starred in time-slip romantic comedy Live Up to Your Name alongside Kim Nam-gil.