In 2011, Seunghoon successfully auditioned for SBS K-pop Star, an entertainment industry audition program. The program's aim is to cast new "stars" into K-pop's three biggest companies: SM, YG, and JYP. The show began to air in December of that year. He successfully made it through to the top 10 before being eliminated in the "7th Live Round" on April 15, 2012, placing fourth overall. Seunghoon appeared on the May 8 episode of SBS "Strong Heart" with other finalists.
On May 16 it was announced that Seunghoon had signed an exclusive contract with YG Entertainment and would shortly thereafter become a trainee. He performed at 'Youth Festival' in Yeouido, Seoul on May 19. Seunghoon was photographed by Moke Najung for a spread in Vogue Girl Korea's June 2012 edition entitled "Boys Like Girls" with fellow K-Pop Star contestant Park Jae Hyung(박제형).
On February 25, 2013 it was announced the Seunghoon would debut as part of a new YG group. On August 1, YG Entertainment's YouTube Channel uploaded a dance performance by a male trainee group. A new trainee "survival" program was announced to begin on the 23rd on Mnet. Called "WIN: Who Is Next?" the program divided the male YG trainees into two groups to compete for a chance to debut. Seunghoon was part of five-membered "Team A."  Seunghoon made his first public appearance as part of Team A at the WIN press conference held on August 20. On October 25, Team A won the third and final battle of "WIN." They took the name "Winner" for their debut. Seunghoon made his first public appearance as a member of Winner in their first stage performance on November 15 in Saitama, Japan when Winner opened for Big Bang at Seibu Dome. Winner officially debuted August 6, 2014.