Six months after debuting as soloist, Gi-kwang was chosen to debut as member of Beast, originally known as B2ST, an acronym for "Boys 2 Search the Top".
However, their name was changed to BEAST (acronym for "Boys of East Asia Standing Tall").
The group debuted in October 2009.
Gi-kwang made his acting debut in the MBC sitcom High Kick Through the Roof, which aired in 2009 and 2010.

In 2010, Gi-kwang featured in K.Will's music video for the song, "Present".
He was also selected as the MC for MBC's Sunday Sunday Night segment Hot Brothers and the KBS talk show Win Win.
In May, Gi-kwang was chosen as the model for the 'Ice Tea Tio' from DongSuh Foods.
In August, Gi-kwang featured on Ahn Jinkyung's single "Love is Pathetic" from the mini album Be the Voice as a rapper.
He, Yoon Doojoon and IU appeared as special hosts on Music Core on August 21, 2010.
On December 29, Gi-kwang was awarded with Popularity Award from the 2010 MBC Entertainment Awards.

Gi-kwang, together with IU, were selected to be new MCs of Inkigayo in replacement of CNBlue's Jung Yonghwa from March 20, 2011.
The two were paired with the existing MCs, 2AM's Jo Kwon, and f(x)'s Sulli.
He was also chosen as the leading man in A Pink's music video, "I Don't Know" Sunny Hill's music video for "Midnight Circus".
In July, Gi-kwang was featured on Brave Brothers' song "Break Up" alongside Electroboyz, as a part of Brave Brother's sixth project album on July 12, 2011.

Also in 2011, Gi-kwang was cast in the romantic comedy drama My Princess starring Song Seung-hun and Kim Tae-hee; and played Lee Ji-ah's patrol partner in the drama Me Too, Flower!.
Gi-kwang won the Best New Actor award at the SBS Drama Awards for his performance in My Princess.
  
On February 8, 2012 Gi-kwang appeared in Ailee's debut music video, "Heaven".
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.

In 2013, Gi-kwang had a role as a terminally ill patient with leukemia on KBS Drama Special My Friend is Still Alive.
In 2014, he starred in the mobile drama 20's (Twenty Years Old).

In 2015, Gi-kwang was cast in SBS's drama Mrs. Cop as the youngest detective on the team.
He next starred as the younger version of Kang Ji-hwan's character in the 2016 MBC drama Monster, and the web drama Momin's Room.
Six months after debuting as soloist, Gi-kwang was chosen to debut as member of Beast, originally known as B2ST, an acronym for "Boys 2 Search the Top". However, their name was changed to BEAST (acronym for "Boys of East Asia Standing Tall"). The group debuted in October 2009. Gi-kwang made his acting debut in the MBC sitcom High Kick Through the Roof, which aired in 2009 and 2010.
In 2010, Gi-kwang featured in K.Will's music video for the song, "Present". He was also selected as the MC for MBC's Sunday Sunday Night segment Hot Brothers and the KBS talk show Win Win. In May, Gi-kwang was chosen as the model for the 'Ice Tea Tio' from DongSuh Foods. In August, Gi-kwang featured on Ahn Jinkyung's single "Love is Pathetic" from the mini album Be the Voice as a rapper. He, Yoon Doojoon and IU appeared as special hosts on Music Core on August 21, 2010. On December 29, Gi-kwang was awarded with Popularity Award from the 2010 MBC Entertainment Awards. 
Gi-kwang, together with IU, were selected to be new MCs of Inkigayo in replacement of CNBlue's Jung Yonghwa from March 20, 2011. The two were paired with the existing MCs, 2AM's Jo Kwon, and f(x)'s Sulli. He was also chosen as the leading man in A Pink's music video, "I Don't Know" Sunny Hill's music video for "Midnight Circus". In July, Gi-kwang was featured on Brave Brothers' song "Break Up" alongside Electroboyz, as a part of Brave Brother's sixth project album on July 12, 2011. 
Also in 2011, Gi-kwang was cast in the romantic comedy drama My Princess starring Song Seung-hun and Kim Tae-hee; and played Lee Ji-ah's patrol partner in the drama Me Too, Flower!. Gi-kwang won the Best New Actor award at the SBS Drama Awards for his performance in My Princess.   
On February 8, 2012 Gi-kwang appeared in Ailee's debut music video, "Heaven". 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. 
In 2013, Gi-kwang had a role as a terminally ill patient with leukemia on KBS Drama Special My Friend is Still Alive. In 2014, he starred in the mobile drama 20's (Twenty Years Old). 
In 2015, Gi-kwang was cast in SBS's drama Mrs. Cop as the youngest detective on the team. He next starred as the younger version of Kang Ji-hwan's character in the 2016 MBC drama Monster, and the web drama Momin's Room.
Lee Gi-kwang