Um began his career by joining Educational Broadcasting System's voice acting division in 1998. Before making his debut as a voice actor, he used to work in theaters, including performing his roles as a Japanese assassinator and a royal servant in a 1997 South Korean musical The Last Empress, and working as the assistant Director for South Korean musicals Gaeddongi and Moskito the same year. In 1992, he also appeared in a South Korean independent film Opening the Closed School Gates.
After his exclusive work at EBS, Um has been a freelancer since 2001. In the early 2000s, he came to prominence after voicing Kira Yamato on the Korean dub of Mobile Suit Gundam SEED. Since then, Um has dubbed a number of television animation series such as Chrono Crusade, Digimon Frontier, Fullmetal Alchemist, InuYasha, and School Rumble. As a result of his popularity among children audiences, the voice actor won a 2012 Nickelodeon Korea Kids' Choice Award for Favorite Voice from an Animated Movie (known as Best Voice Actor/Actress in South Korea). He became the recipient of the award for his role as Po in Kung Fu Panda: Legends of Awesomeness, gaining 3,121 votes, which is about 38% of the entire poll.