Kim Sung-kyun began acting in theater plays before expanding his craft to television and films. In 2012, he made his big screen debut in the movie Nameless Gangster: Rules of the Time, where he portrayed a loyal henchman to his gangster boss. He picked up three Best New Actor trophies for his performance in the film, including one from the 48th Baeksang Arts Awards. He next starred in the suspense thriller movie The Neighbor, which won him three more Best New Actor awards. In 2013, he became part of the tvN series Answer Me 1994, which earned him a Best New Actor for TV nomination at the 50th Baeksang Arts Awards. In 2015, he was cast in Answer Me 1988, which reached a peak audience rating of 18.8 percent for its finale episode. He received the Scene-Stealer Award at the 1st tvN10 Awards for his work in the drama. In 2016, he appeared in the SBS sageuk Moon Lovers: Scarlet Heart Ryeo, in which he was nominated for a Special Acting Award at the SBS Drama Awards. In 2019, he played the role of a clumsy detective in the hit SBS series The Fiery Priest, in which he took home an Excellence Award at the SBS Drama Awards.

Kim Sung-kyun and his wife have been together since 2010. They have three children.