Seo Kang-joon debuted with the five-member actor group 5urprise in September 2013 along with Yoo Il, Gong Myung, Kang Tae-oh, and Lee Tae-hwan. Unlike other idol groups, 5urprise began their career with an acting project before showcasing their music. The group made their acting debut in the 12-episode web series After School: Lucky or Not. 5urprise then released the single Hey U Come On for the drama’s soundtrack. The group also have three single albums together.

Seo Kang-joon made his solo acting debut in the 2013 SBS series Suspicious Housekeeper. In 2014, he became part of the main cast of the MBC romantic comedy Sly and Single Again, which won him Best New Actor at the 7th Korea Drama Awards. That same year, he was nominated for Best New Actor at the KBS Drama Awards for What’s With This Family. In 2016, he joined tvN’s Cheese in the Trap, which earned him the Best Entertainer Award at the 1st Asia Artist Awards. Also in 2016, he was cast in his first leading role in the tvN drama Entourage, which was adapted from the American TV series of the same name. In 2018, he played a dual role in KBS2’s Are You Human Too, which gave him an Excellence Award at the KBS Drama Awards.

Seo Kang-joon studied in Malaysia for over a year when he was a young teen. He then attended Dong Seoul College’s Faculty of Performing Arts. He enlisted for his mandatory service in November 2021.