He was then scouted on the street, while working part-time, by a talent manager who liked his refined appearance, henceforth choosing acting as his career.

In Kim's 2005 film The Bow, he played a college student involved in a mysterious and dangerous relationship between an old man and a 16-year-old girl.

That same year, Seo was cast as the smart and cold Park Yoon-hoo in the 167-episode daily drama Hearts of Nineteen, leading the series alongside Ku Hye-sun.

[5][clarification needed] Seo returned to acting by playing in an independent film The Inside Story of Him and Her and appearing as a cameo in High Kick Through The Roof.

In 2010, Seo was cast as a lead role in the series OB & GY,[6] followed by weekend drama Gloria opposite Bae Doona.

[12] From 2013 to 2015, Seo appeared in two long-running variety programs; Cool Kiz on the Block,[13] and Law of the Jungle which was shot in Costa Rica.