Yoo was born as Ahn Yeon-seok on April 11, 1984 in Jinju South Gyeongsang Province, South Korea. In his junior year in high school, Yoo decided to take up acting and followed his brother, who was studying another year to enter university, to Seoul and they lived together near the district of Samseong-dong.
While taking acting classes, Yoo met a friend who would later work on the costume staff of film director Park Chan-wook. Remembering that he resembled actor Yoo Ji-tae, he gave him a call about auditioning for a role. Without having to pass out his resume, he made his acting debut as Yoo Ji-tae’s younger counterpart in the acclaimed 2003 film Oldboy.
Upon graduation from Sejong University with a degree in Film Arts, he resumed his acting career in 2008 and began using his agency’s recommendation stage name, Yoo Yeon-seok.