Before becoming an actor, Kim Sang-joong was enlisted in the Marine Corps. But after graduating from Dongguk University with a degree in Theater and Film, he embarked on an acting career at age 25. Kim made his acting debut in the stage play I Love Bread in 1990, and was one of the co-founders of the theater troupe Extreme Legend. He soon transitioned to film and television, and became known for his distinctive voice and confident and charismatic acting, often as morally ambiguous characters. Among Kim’s notable roles in television dramas include a middle-aged man having an affair with his wife’s best friend in My Husband’s Woman (2007), the titular besieged monarch in Eight Days, Assassination Attempts against King Jeongjo (2007), a former agent who trains his foster son as a weapon in his plans for revenge in City Hunter (2011), and a presidential candidate willing to go to any lengths to win, including murder and cover-up, in The Chaser (2012).

On the big screen, Kim was praised for his comic acting in the gangster comedy My Boss, My Teacher (2006), and has also starred in arthouse films directed by Hong Sang-soo, such as The Day He Arrives (2011) and Our Sunhi (2013).

Since 2008, Kim has hosted I Would Like to Know, a long-running program with a similar format to Unsolved Mysteries.