He made his acting debut in 1995 in the stage play Richard III, though he later became best known as a musical theatre actor, starring in Singin' in the Rain,[2] The Sorrows of Young Werther,[3] Grease, The Three Musketeers,[4] Jack the Ripper,[5] The Count of Monte Cristo,[6][7] Catch Me If You Can,[8][9][10] and Rebecca.

[11][12] Um's popularity increased after he began playing supporting roles in television dramas such as Life Special Investigation Team, Worlds Within, Hero,[13] Dream High,[14] Scent of a Woman and Phantom.

After his discharge from the military in 1998, he faced a period of obscurity in Daehakro, taking on various roles in both large and small productions, starting with the musical "Gwanggaeto the Great."

[20] Um also appeared in Kimchi Cheese Smile, as Ki-jun, Shin Goo's eldest son, who rents the second floor of his friend Byeong-jin's house.

He played the role of unknown artist Park Hyeong-joo, who is torn between his beloved wife and his male lover.

[18] In 2009, Um made a return to the theater stage with Maxim Gorky's play The Lower Depths which features characters from the lower echelons of society including a fallen nobleman, a woman suffering from a terminal lung disease, and an alcoholic actor, he takes on the role of Satine, a once-intellectual turned conman.

[29] In 2020, he appeared in TV series The Penthouse: War in Life as Joo Dan-tae, the main villain and a scheming real estate genius.