Koo is one of the first-generation Hallyu stars when CLON gained huge popularity internationally, most notably in Taiwan.

Koo attended Kyunggi High School, where he met fellow CLON member Kang Won-rae.

[3] In 1990, Koo and his best friend from Kyunggi High School Kang Won-rae joined a dancing competition, which they won fourth and first place respectively.

[6] In November 2000, fellow CLON member Kang Won-rae got into a tragic motorcycle accident which left him paralyzed from the waist down.

Koo took a long hiatus from his musical career to take care of his best friend till Kang was able to sit in a wheelchair on his own.

After a long hiatus from his music career, Koo released his self-titled debut studio album in November 2003.

"Escape" was nominated as "Best Buzz Asia South Korea" at the 2004 MTV Video Music Awards Japan.

Koo performed at the 2003 Mnet Asian Music Awards, including songs like "You're My Life", "Escape", "I", and "Thinking About You".

In 2005, Koo reunited with Kang Won-Rae and released their fifth album, Victory (2005), with "My Dear Love, Song" as its lead track.

In addition, Koo along with singers Jang Woo-hyuk, Lee-Min-woo, and Kang Won-rae performed a dance medley at the 2005 Mnet Asian Music Awards.

On August 13, 2011, Koo released his second extended play Comeback Remix, which he was in charge of arranging and producing.

In the same year, he attended the 2013 KCON, an annual Korean wave convention, (aka "M Countdown What's Up LA") in Los Angeles.

In 2014, Koo was featured in singer IU's cover version of Clon's 1996 hit song "Kungtari Shabara", titled "Boom Ladi Dadi", alongside Kang Won-rae.

Other songs include "Go Tomorrow", which featured the innovative genre of EDM, "Ore Ore O", a remixed version of Clon's classic hit "First Love", and  "90's DJ Koo Driving Mix", a lengthy track that is a mishmash of Clon's 14 hits.

[18] He and Kang Won-rae reunited as CLON at the opening ceremony and performed "Go Tomorrow" and "Kungtari Shabara" at the Pyeongchang Olympic Stadium.

In 2020 and 2021, Koo played at the Seoul World DJ Festival which were held online both years due to the Covid-19 pandemic.

Koo first met Taiwanese actress Barbie Hsu in August 1997 when he was invited to Taiwan as a concert guest.

[20] After meeting again in August 1998 when Koo went to Taiwan to promote his album, the pair immediately fell in love and began dating shortly after.

Due to a "dating ban" in the K-pop industry, their relationship was opposed by Koo's management and they were forced to break up in 2000.

[20][22] The couple married on February 8, 2022 in Seoul, South Korea; they also registered their marriage in Taipei, Taiwan, on March 28, 2022.

[25] Koo splits his time between homes in the UN Village neighborhood in Hannam-dong, Seoul, South Korea,[26] and Taipei, Taiwan.

In 2022, Koo published a limited edition NFT artwork in partnership with Kakao Ground X Clip Drops which sold out in 7 seconds.