Yoo was born in Incheon and raised there by his family and the younger of two siblings. He made his entertainment debut in a cellphone commercial in 1999 after his mother sent in a photo of her son to an ad agency. In 2000, Yoo began his career as a child actor, first appearing in television dramas. He rose to stardom in his first film The Way Home, playing a bratty city boy who learns to appreciate country life when he's forced to spend the summer with his deaf-mute grandmother. The low-budget film was a surprise box office hit in 2002, drawing more than 4 million admissions.He was then affectionately labeled as "Nation's Little Brother". Thereafter, he also starred in animal movie Heart Is... (2006) about a boy and his beloved dog, and Unforgettable (2008) about school children from a remote island who go on a field trip to a candy factory in Seoul during the 1970s. He continued acting in television, appearing in Magic Warriors Mir & Gaon (2005), an adventure series for children. Yoo further built his filmography, playing younger counterparts of male protagonists in television dramas, including general Yi Sun-sin in Immortal Admiral Yi Sun-sin (2004), King Seongjong in The King and I (2007), and Gwanggaeto the Great in The Legend (2007). In 2009, he starred in the films City of Fathers and 4th Period Mystery. He also appeared as Kim Chunchu in the period epic Queen Seondeok.