In October 2017, he joined survival program The Unit, in which he finished in first place and became a member of the project group UNB.

On May 15, 2014, NH Media announced, via U-KISS's official Korean website, new member Jun to the public.

On April 10, 2019, Lee released his first Japanese solo single "Phenomenal World," which explored the futurepop genre.

[9][10] Tower Records Japan ranked "Phenomenal World" as the #7 best-selling Japanese single by a Korean artist for the first half of 2019.

After a digital download was released on October 30, the Japanese version ranked #2 on the dwango.jp K-POP Life weekly singles chart.

[17] On December 5, he made his solo debut in Korea with the title track "Curious About U" and the single album Gallery.

After departing NH Media in August, he announced in November that he co-founded a new company J-flex, which will serve as his current agency for his acting projects.