Kim Hyang-gi began her career as a child actress, and first appeared in the animal film Ma-eum-i... (2006), alongside Yoo Seung-ho in 2006.

In 2013, Kim starred in the television drama Yeowangui Gyosil (2013), which won her a Best Young Actress Award at the 2013 MBC Drama Awards. Kim next appeared in Uahan Geojinmal (2014), a film adaptation of Lyeo-ryung KimKim Ryeo Ryeong's novel, and her performance as a victim of bullying won her the Best New Actress award at the Baeksang Arts Awards.

In 2015, Kim had a lead role in the Drama City television special, Nun-gil (2015). The two-part drama series is about the "comfort women" in Korea under Japanese rule during World War II, and was later released as a film in theaters.

In 2017, Kim starred in the web drama Boksoonoteu (2017), which earned positive reviews and had over 11 million views online. The same year, she starred in the fantasy blockbuster Sin-gwa ham-kke: In-gwa yeon (2018). The film was the second highest-grossing film in South Korea, and Kim won the Best Supporting Actress award at the Blue Dragon Awards. Kim subsequently reprised her role in the sequel of the film, Sin-gwa ham-kke: In-gwa yeon (2018).

In 2018, Kim starred in the drama film Young-ju (2018), playing a lonely girl whose environment has forced her to grow up too fast. The same year, she was cast in the two-episode romance drama Drunk in Good Taste (2018).

In 2019, Kim starred alongside Jung Woo-sung in the drama film Jeungin (2019). She earned positive reviews for her portrayal of an autistic girl, and was nominated for Best Actress in the 55th Baeksang Arts Award.

She also won the Best Actress for her role in Innocent Witness at the 39th Golden Cinema Film Festival.