Kim Sung-kyu made his acting debut in 2011 through the short film Nineteen Ninety Nine and the theater play 12 Angry Men. He then had appearances in the 2014 heist film The Con Artists and the 2016 survival drama movie Tunnel. His career breakthrough came in 2017 when he became part of the hit action thriller movie The Outlaws, which earned him a Best New Actor nomination at the 54th Baeksang Arts Awards. In 2019, he acted alongside Joo Ji-hoon and Bae Doo-na in the Netflix zombie sageuk Kingdom and went on to reprise his role in its sequel. Also in 2019, he starred in the action thriller film The Gangster, the Cop, the Devil, which earned him the Best New Actor trophy at the 40th Golden Cinema Film Festival. In 2021, he joined the main cast of Coupang Play’s One Ordinary Day.

Kim Sung-kyu attended Daejin University.