Seo was born on October 23, 1987, in Ulsan. He grew up in relative poverty, his mother worked as a recyclables collector and his Father a welder. He has a younger sister. Seo trained as a ssireum Wrestler, learnt boxing and mixed martial arts in school and is a Hapkido 2nd dan. He decided to become a singer at the age of ten, after being inspired by the Korean rock musician Kim Jung-min. He performed at family gatherings and school events, and went on to study Applied Music at Daebul University, also auditioning in entertainment agencies. After repeatedly being rejected and told to lose weight, he struggled with bulimia for a time.
In 2009, Seo competed in the Mnet singing contest Superstar K, winning the show's first season. The subsequent spotlight came with a hectic schedule of recording, photoshoots, interviews and television appearances as well as chart-topping singles and albums. Seo also faced negative consequences of media attention, including rumors of plastic surgery and "cold shoulder" treatment from the three major Korean networks, as he was a cable television star.
In 2012, Seo was part of the cast of the jukebox musical Gwanghwamun Love Song and made his onscreen acting debut with a supporting role in KBS's Love Rain. He also signed with Japanese agency Irving Entertainment. He landed his first leading role later that year, in cable channel tvN's '90s nostalgic hit drama Reply 1997. Seo's portrayal of a teenage boy in unrequited love with his childhood best friend won praise from both audiences and critics. He also recorded two singles for the Soundtrack (both duets with co-star Jung Eun-ji), and one of them, "All For You", became one of the best-selling singles of the year on the Gaon Single Chart. Shortly after, he appeared in MBC's weekend drama The Sons (also known as Rascal Sons).
Seo joined the newly launched reality/variety show I Live Alone from January to June 2013. He returned to the music scene in April 2013 with the single With Laughter or with Tears, the first soulful ballad from his discography. Several weeks later, the song was followed by his Japanese debut with the release of the single Fly Away.
In 2014, Seo released his first album titled Everlasting, which comprises his previously released singles “Fly Away” and “We Can Dance Tonight”. The album also features a new track titled “Everlasting Love” with a total of 12 tracks. The album debuted at number nine on the Oricon Daily Chart. Seo later won the Best 3 New Artists (Asia) award at the Japan Gold Disc Awards.
In 2015, Seo joined the cast of SBS variety show Law of the Jungle from March to May, for the Indochina season. He was then cast as a genius profiler in the police procedural/romance series Hello Monster which premiered in June 2015 on KBS2. Later that year, he participated alongside other artists on Jellyfish Entertainment's collaborative track “Love In The Air” (Hangul: 사랑난로) for the album Jelly Christmas 2015 – 4랑.
In 2016 Seo joined Law of the Jungle for the second time for the show's Mongolia season. He also starred in OCN's workplace drama 38 Revenue Collection Unit, playing a professional swindler. The drama became the highest rated drama for OCN, and Seo won a special award or his role at the 11th Asian TV Drama Conference. He then took on the leading role in MBC's romantic comedy Shopaholic Louis.
On December 2017, it was confirmed that he has been dating Park Bo-ram since 2015.