She gained widespread recognition in the television dramas Hearts of Nineteen (2006), The King And I (2007), Boys Over Flowers (2009), Take Care of Us, Captain (2012), Angel Eyes (2014), Blood (2015).

She used to be a trainee under SM Entertainment, before switching over to DSP Media and preparing to debut in a girl group called Ria.

[3] Koo rose to fame for her role as Geum Jan-di in the mega hit KBS2 drama Boys Over Flowers opposite Lee Min-ho,[4] which gained her pan-Asia popularity.

[13][14] After two years break from acting career, Koo appears in MBC channel's two-part documentary drama Heo Nanseolheon.

[15] Koo then made a small screen comeback acting in SBS's weekend drama Angel Eyes in 2014.

She played a legally blind woman who gets her vision back in an operation and decides to embrace life with passion by becoming an emergency rescue worker.

[citation needed] She then released her self composed and written digital single "Brown Hair" in 2010, with the music rearranged by bossa nova pianist Choi In-young.

[citation needed] On April 28, she released her first regular album And Spring that consist of 11 tracks including her singles "Stupid", "Brown Hair", and "It's You.

Fragment of Sweet Memories was selected as one of Korea's outstanding 3D films, and was shown at the Cinemountain in Busan Cinema Center during the International 3D Festival (I3DF), which Koo serves as the promotional ambassador of.

[56] Not only did she took part in producing, script-writing and directing of the movie, she also plays the main character as a mother who gives her daughter an oppressive corporal punishment.

[60] Aside from her acting and directional works, Koo participates in a wide range of projects, including writing and drawing as well as ambassador activities.

[62][63] In 2009, Koo published her first illustrated novel Tango (탱고), centered around a 20-something young girl who goes through two separate relationships to finally mature into a woman.

[64] Its release coincided with her first solo art exhibition, also titled "Tango" which was held in July 2009 at La Mer Gallery and attracted a total of 10,000 visitors.

[66] She donated all proceeds from the sale of her artwork toward purchasing "clean cars" (germ-free automobiles) for the Korea Leukemia Patient Group.

[72] The illustration poster of her cat 'Mango' will be sold at this exhibition, where the profit will be donated to charity organization ‘Community Chest Hong Kong’.

[79] The pair appeared in the reality show Newlywed Diary produced by Na Young-seok, which showcased their married life.

[81] On August 18, 2019, Koo posted an image of text messages between her and her husband Ahn Jae-hyun discussing divorce to her Instagram account.

[85] On September 1, 2019, Koo revealed in Instagram post that she would be taking a break from the entertainment industry to resume college.

[86] On July 12, 2022, Koo made a donation to her alma mater, Sungkyunkwan University, to help the needy through the Senior Love Learning Support Fund.