Seo Kang-joon was born Lee Seung-hwan in Gunpo, Gyeonggi Province, South Korea.

[3] Seo took on bit parts in various television series, before making his acting debut as with a supporting role in SBS' drama The Suspicious Housekeeper.

[16][17][18] The same year, he starred in his first leading role in tvN drama Entourage, the Korean remake of the American television series of the same name.

[26] In 2024, Seo was cast in the MBC action comedy Undercover High School,[27] which aired in the first half of 2025.

[29] Seo received the division commander's commendation for excellent performance in basic military training.