Onew was discovered at the 2006 S.M. Academy Casting. Onew received great appreciation from Lee Soo-man, SM Entertainment's founder, at Girls' Generation's debut showcase. Lee Soo-man noticed him and wanted to hear Onew sing during a one time on the spot audition. He signed the contract with SM the day after he auditioned.
In 2008, he was chosen as a member of the group Shinee, and the 5-member boy group debuted on May 25, 2008 on SBS's Inkigayo.
Onew contributed to singles for Lee Hyeon-ji (for "Vanilla LOVE" and "Vanilla Love Part 2"), Girls' Generation member Jessica for the song "One Year Later" in the Tell Me Your Wish (Genie) mini-album and a duet with Kim Yeon-woo for the song, "The Name I Loved" from the third mini album by Shinee, "2009, Year Of Us." 
Onew starred in various musicals starting in 2010, debuting in the musical Hyeongjeneun Yonggamhaetda (형제는 용감했다, "Brothers were Brave") alongside singer Lee Ji-hoon. He then starred in the Korean production of Rock of Ages playing the lead role, Drew.
He has also showcased his acting skills via television having a cameo role as a clumsy doctor in the drama Dr. Champ. He appeared in the drama's final episode. He also had cameo role's in the dramas Athena: Goddess of War, Oh My God x2,Pure Love and for sitcom Royal Villa.
In 2015 Onew was chosen out of more than 2,000 people as the main lead of a new web drama, Dating was the Easiest. The drama is adapted from a popular webtoon of the same title, the drama revolves around the pure yet boisterous romance between a tiger spirit of the mountains and a high school girl. Onew was casted because of his unique analysis of the character and his determination whilst preparing for the auditions. He was also confirmed for the upcoming Song Hye-kyo's and Song Joong-ki's Descendants of the Sun drama. He's going to play a young doctor, who is a junior of Song Hye-kyo's character, Kang Mo-yeon.
In addition to music, Onew became an MC on two programs — KBS's Yahaengsung (야행성, "Nocturnal") and MBC's Show! Music Core. In January 14, 2014, his participation in the Borneo edition of the reality showLaws of the Jungle was confirmed. He was also in the cast for the Brazil edition, but had to leave and be substituted because of schedule conflicts.
Onew graduated from the Gwangmyeong Information Industry High School. In his senior year in high school, Onew had ranked as the second top student in his grade. He is currently attending Chungwoon University, his major being broadcasting music.
Onew underwent a vocal cord polyp removal and vocal fold mucosa reconstruction operation on June 3 at a hospital in Seoul. Kim Yeon Woo, a friend of Onew as well as his vocal coach revealed during the December 17 broadcast of MBC FM4U’s FM Date radio that Onew's voice got better after the vocal cords surgery. He also said that Onew's vocal range "improved and he can make sounds comfortably too".
Fans of Onew showed up to one of Onew's performances in "Rock of Ages" with 1.44 tons of rice. Onew donated the 1.44 tons of Dreame rice to help feed North Korean children, and according to a Dreame representative, the rice was delivered to an Incheon storehouse on the 16th October 2010 and was sent to North Korea on the 22nd. Onew had also donated 770 kg of Dreame Rice to the Children in Need in South Korea in May 2010 when he was in the "Brothers are Brave" musical.