Junho first gained public attention when he won Superstar Survival in 2006. The show started with twelve teenage competitors and, as the weeks went on, they were eliminated one by one until three were left. The process followed a similar style to that of reality show Survivor. Junho signed a contract with JYP Entertainment after winning the contest, placing first out of 6,500 competitors. Future 2PM bandmates Taecyeon and Chansung had also competed.
In 2008, he took part in Mnet's Hot Blood Men in which 13 people train to join a boy band, One Day. That band split into two, 2AM and 2PM. Junho joined the latter.
In 2012, Junho coupled with actress Kim So-eun on MBC's Music and Lyrics for the song "Sad Love", written by Kim So-eun and composed by Junho. He also collaborated with Vanness Wu in the song "Undefeated" for Taiwanese drama Ti Amo Chocolate soundtrack, and composed "Forever" for 2PM's Japanese single Masquerade. In 2013, Junho participated as a composer for fellow member Jang Wooyoung's debut EP 23, Male, Single with the song "Be With You".
In July 2013 Junho released a solo mini album, Kimi no Koe. The homonymous song was released on July 8, and the next day Junho began his first solo tour, which ended on August 29 in Tokyo. On July 9, 2014, Junho released his second Japanese mini-album, FEEL, for which he wrote, composed, and produced all the tracks. FEEL topped Oricon's daily album chart the day of its release. From July 3 to August 13, Junho embarked on his second solo tour in Japan. On July 15, 2015, Junho releases his third Japanese mini-album, SO GOOD, which he promoted in Japan from 7 July to 5 August.