He originally debuted as solo singer with the stage name AJ (Ace Junior), releasing his first mini album First Episode: A New Hero on April 4, 2009.

In October 2009, he debuted as the main dancer, visual and a lead vocalist of boy group Beast, which was renamed Highlight in February 2017.

He was asked by the former president of Cube Entertainment to join the agency without auditioning, making him the company's first trainee.

He was known as the solo singer AJ or Ace Junior when he debuted and released a mini-album First Episode: A New Hero on April 4, 2009.

[3] However he met controversies when his name created dispute among netizens for having a similar name with Paran's AJ (who later became a U-KISS member).

[7] Gi-kwang made his acting debut in the MBC sitcom High Kick Through the Roof, which aired in 2009 and 2010.

[12] In August, Gi-kwang featured on Ahn Jinkyung's single "Love is Pathetic" from the mini album Be the Voice as a rapper.

[15] Gi-kwang, together with IU, were selected to be new MCs of Inkigayo in replacement of CNBlue's Jung Yonghwa from March 20, 2011.

[22] Gi-kwang also hosted SBS K-Pop Super Concert in America on November 10 alongside Tiffany Hwang and Jung Yong-hwa and Melon Music Awards with Infinite's Woohyun, Moon Hee-jun, and Danny Ahn.

[23] In 2013, Gi-kwang had a role as a terminally ill patient with leukemia on KBS Drama Special My Friend is Still Alive.