[1][2] He began his professional career as an actor in local television series, receiving recognition for his roles in The Condor Heroes 95 (1995), Detective Investigation Files IV (1999), and A Step into the Past (2001).

Koo started gaining wide recognition in 1995 with his leading role in a costume drama The Condor Heroes 95, a popular series based on a classic wuxia novel.

This deviation from his usual cool aloof on-screen persona to a funnier, carefree and down-to-earth one resonated with the audience and widened his acting range.

Due to the popularity of TVB dramas in the late 1990s, Koo amassed a sizeable fan base in Hong Kong, Taiwan, mainland China and South-east Asia.

Koo acted as a mute boy living on rural island in romantic literary film Sealed with a Kiss (1998).

In 2004, Koo took on the role of Sze-to Bo, a perpetually drunk and despondent former judo champion with hereditary eye disease, in Johnnie To’s Throw Down which was premiered at the 61st Venice International Film Festival.

In a female perspective romantic film Happy Birthday (2006) adapted by Sylvia Chang from 'Rene Liu's novella, Koo portrayed a sensitive man who continued to take care of his ex-girlfriend after his death.

[8] Koo also took supporting roles in action movies, such as Jackie Chan's Robin B Hood (2006), Donnie Yen's Flash Point (2007).

His character as an unscrupulous drug addict in Derek Yee's Protégé (2007) differed from his previous performances, as he forwent his suave image for an emaciated look.

In the remake of Cellular, Connected (2008) directed by Benny Chan, he played an average Joe[10] cast in a race against time to save a stranger.

In Overheard (2009) directed by Alan Mak and Felix Chong, he put on 30 pounds’ weight to take the role of a middle-aged cop and father of four more convincingly,[11] and earned Best Actor nominations in Hong Kong Film Critics Society Awards.

In Soi Cheang's SPL II: A Time for Consequences (2015), Koo took the role of an ill and evil mastermind of a criminal organization.

In Paradox (2017), Koo played a cop who was determined to avenge his daughter, and performed most of the film's action sequences without the use of a stunt double.

In 2015, Koo joined the board of Fresh Wave International Short Film Festival[17] held in Hong Kong.

[18] Koo has published several books over the years including his photo albums Present (1999) and Cool (2001), as well as his autobiography Find Happiness (2002).

With a career spanning two decades, Koo has endorsed a large number of brands in Hong Kong and Mainland China.

Some of the major brand endorsements include, Pepsi,[23] Brooks Brothers, Samsung Galaxy,[24] Xbox 360 Kinect,[25] Toyota Corolla, Yokohama Tire,[26] Lay's, Lipton, Lotte chewing gum, Tropicana,[27] Gillette, OSIM,[28] Tag Heuer, Epson printer, Matisse, Johnnie Walker, Pentax camera, Clinique, Hugo Boss Perfume, Bally, Puma, DBS Black World MasterCard.

In 2018, 55382 Kootinlok, a main-belt asteroid discovered by Bill Yeung at the Desert Eagle Observatory in 2001, was named in honor of Koo.