[4] He decided to become a singer at the age of ten, after being inspired by the Korean rock musician Kim Jung-min and watching him perform ''Sad Promise''.

[5] Seo performed at family gatherings and school events,[6] and moved to Seoul alone at the age of 20 to pursue his dream of becoming a singer despite his father's objections.

[7] While studying Applied Music at Daebul University, Seo auditioned for various record agencies such as JYP Entertainment and DSP Media but to no success.

Having lost over 33 pounds (15 kg) since his debut through exercise and strict diet control, Seo's changes in his physical appearance brought about rumors of plastic surgery.

[22][23] In 2011, shifting his sound, Seo released the single ''Broken'' incorporating choreography into his performance, as well as summer dance track ''Shake It Up'' in which he penned the lyrics for.

[24] Despite a successful debut, Seo reportedly received the "cold shoulder" treatment from the three major Korean public networks, as he was a cable television star.

[44][45]In June 2013, he was cast in Master's Sun, a drama series written by the Hong sisters, playing a former soldier who served in the Zaytun Division who becomes the head of security for a shopping mall company.

[46][47] In October 2013, he starred in his first big-screen leading role in No Breathing, a coming-of-age film about two swimming rivals, alongside Lee Jong-suk and Kwon Yuri.

[48][49] He then played a dual role in Another Parting, a five-episode miniseries about a man and a woman who meet in the final moment of their lives and spend one special day together.

[53] Following the cancellation of action noir film Wild Dog which would have been Seo's second big-screen appearance,[54] Seo headlined cable romantic comedy series High School King of Savvy in which he played a high school student and star hockey player who leads a double life when he's forced to take on his elder brother's job as an executive at an IT company.

[62][63] Seo called the role a turning point in his acting career,[64] and he received favourable press reviews for his ease of transformation into multiple character personas.

[73] In 2018, Seo starred in mystery melodrama The Smile Has Left Your Eyes, a remake of the 2002 Japanese television series Sora Kara Furu Ichioku no Hoshi.

[77][78][79] The Straits Times wrote, "It is a role that could easily slip into creepiness or smugness but Seo manages to pull off a delicate balancing act.

[82] In the same month, he also returned to the big screen after eight years in Yoo Ha's comedy-crime film Pipeline as a drilling expert involved in an oil heist.

[91] Seo then starred in Kim Hong-sun's action thriller film Project Wolf Hunting as a criminal with Jang Dong-yoon, alongside Choi Gwi-hwa and Sung Dong-il.

[93] In February 2017, it was announced that Seo received his official military enlistment date, set to take place on March 28.