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 which follows the extreme training of 13 trainees in order to become a member of the boy band One Day. However One Day was split and spawned two boy bands, 2AM and 2PM, of which the latter one Junho became a member. Six months after Hot Blood Men was aired on TV, 2PM debuted with their first single "10 Out of 10" from their first EPHottest Time of the Day, but it was their second EP 2:00PM Time For Change which skyrocketed their success in the Korean music industry.
Following Jaebom's controversy and departure in 2009, all 2PM members, including Junho, were withdrawn from their regular appearances on variety shows. Although he was supposed to be a regular member of Dream Team 2, Junho was only part of the pilot episode; however, in March 2010, he returned as a permanent cast member of the show. In August 2010, Mnet created a list of the top thirty talented idols who have beautiful voices, and Junho took 19th place.
Beginning in 2011, Junho took a more proactive role in music composition. He contributed to the song "Give It to Me" from Hands Up album, released on July 21, which was featured in the action thriller movie Blind. He also performed his self-composed song "Move On" on Hands Up Asia Tour, which was included in 2PM's album Best of Korea and then released as a ringtone on Japan's biggest digital website Recochoku, ranking first on the service's daily chart upon its release.
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 solo debut extended play 23, Male, Single with the song "Be With You".
In May 2013, JYP Entertainment revealed that Junho would make his solo debut in Japan with mini album Kimi no Koe, released on July 24. 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, he 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.
Junho is professional baseball player Sun-woo Kim's cousin.
On February 15, 2012, Humanitarian Organization World Vision reported that they launched Global Share Project in collaboration with EBS. Junho was selected as the first person to volunteer and travelled to Ethiopia, where he met with patients suffering from elephantitis. Junho has also been secretly supporting a child in Ethiopia who he got to meet in person.