Moon officially debuted as a comedian in 2002 through the SBS Comedian Contest. In 2003, he joined SBS's comedy show ''''. Moon gained attention for his portrayal of a cross-dressing character in the 'Quiz, Let's Play' segment of the show. His catchphrase, "I don't know~ layo," became associated with his character in that segment. He also participated in other segments of '''', including the 'Fine Words Drama' corner, 'Pigeon Karaoke', and the Pyo's brother. In 2006, Moon Se-yoon appeared in the MBC sitcom
Elephant, directed by Lee Min-ho. In the sitcom, Moon portrayed the character of Baek-soo Joo Bok-soo, the second son of Joo-hyun's character in the landlord's family.
Elephant depicted the intertwined lives of the landlord's family and the tenant's family, exploring themes such as the school lives of teenagers, the romantic relationships of those in their 20s, the challenges of unemployment, and the family dynamics of middle-aged and elderly characters. Moon's performance earned him a nomination for the Excellence Award in the
MBC Entertainment Awards. Moon's performance earned him a nomination for Best New Actor at the 5th
Korean Film Awards. After completing his mandatory military service, Moon was discharged in April 2013. He signed with A-List Entertainment in the same month. Subsequently, he and Jung Ju-rib joined tvN's
Comedy Big League. After joining FNC Entertainment, Moon is active in entertainment show. == Personal life ==