Koo' father was an actor and then changed career to a businessman. Koo was born and received education in Hong Kong. He is an alumnus of St. Teresa's School Kowloon and Carmel Secondary School. During 1991 to 1993, Koo was a modelling agent, sometimes he himself worked as a model for advertisements and karaoke videos.
In 1993, Koo signed as a contract artist with TVB (Television Broadcasts Limited).
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. In 1997, Koo began sporting an extreme tan (for which he is well known for), which apparently enabled him to take on more contemporary roles. In 1999, Koo received TVB's Most Popular Actor Award for Detective Investigation Files IV. In 2000, Koo received TVB's Most Favorite Character Award for At the Threshold of an Era ll. Koo took a different approach to his role in A Step Into the Past. 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. In 2001, Koo was given TVB's Most Popular Actor Award and Favorite Character Award Award once again for A Step into the Past. He then went on an indefinite hiatus from television and has since focused on his film career.
Due to the popularity of TVB dramas in the late 1990s, Koo amassed a sizeable fan base in Hong Kong, Taiwan, China and South-east Asia.
Since 1994, Koo began taking on roles in films, such as the paranormal Troublesome Night series and many action films including Wilson Yip's Bullets Over Summer(1999). Koo acted as a mute boy living on rural island in romantic literary film 'Sealed with a Kiss(1998).
During 2001 to 2004, Koo signed contract with China Star Entertainment Group and starred in many romantic comedies, such as La Brassiere and Naked Ambition.
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 61st Venice International Film Festival. The movie was a marked departure from his usual commercial genres, and he also began his long-term collaboration with To and Milkyway Image.
Koo later received international acclaim for his chilling performance in Johnnie To's Election 2 (a.k.a. Triad Election), which was screened during the "Out of Competition section" at the 59th Cannes Film Festival (Election was competitive film at the 58th Cannes Film Festival). He won the award for Most Beloved Actor in the Hong Kong UA Film Awards 2006 through public voting for his role in Election 2.
In a female perspective romantic film Happy Birthday (2006) adapted by Sylvia Chang from 'Rene Liu's novella, Koo portrayed a sensitive man who took care of ex-girlfriend after death.
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. The supporting role earned him positive critical feedback and Best Supporting Actor nominations in Hong Kong Film Awards and Golden Horse Award.
In Sylvia Chang's female perspective gangster film Run Papa Run (2008), Koo played a reckless triad leader who tried to shield his daughter from his past misdemeanours. Koo earned Best Actor nominations in the Hong Kong Film Awards and Golden Horse Award for this role, which required him to portray a character from his late teens to his fifties. Since then Koo took more chanllegings on father roles and middle-aged roles.
In the remake of Cellular, Connected(2008) directed by Benny Chan, he played an average Joe cast in a race against time to save a stranger.
In Soi Cheang's Accident(2009) which was a competitive film in 66th Venice International Film Festival, Koo acted as a high-strung killer-group leader who was good at disguising murder as accidents.
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 four kids' father more convincingly, and earned Best Actor nominations in Hong Kong Film Critics Society Awards. This film was successful and then made into series with the same cast and directors but different stories.
In 2010 Koo signed pre-paid contracts with Pegasus Motion Pictures, then appeared in its All's well, ends well comedy series, and anti-corruption film series (Z Storm, S Storm).
In costume A Chinese Ghost Story(2011), Koo acted as a Chinese "Van Helsing" fell in love with a fox-fairy and sacrificed himself in the end. Koo made a return to his romantic comedy roots in Don't Go Breaking My Heart(2011) and sequel Don't Go Breaking My Heart 2(2014) directed by Johnnie To and Wai Ka-Hui, acted as a dissolute stock broker.
In Drug War(2012) directed by Johnnie To, Koo took the role of a ruthless and deliberate Hongkong drug lord who dared to do anything to get away from mainland China's death penalty.
Koo received a Hong Kong Film Awards Best Actor nomination again for his undercover cop role in Benny Chan's 2013 film The White Storm.
In Out of Inferno(2013), Koo acted as a resigned firefighter who encoutered high building fire. In Overheard 3(2014), he acted as a revenger with artificial limb. 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 Johnnie To's Three(2016), Koo acted as a cop leader at the edge of crime.
In Shed Skin Papa(2017), a competitive film in 29th Tokyo International Film Festival, Koo acted as a frustrated director understood his farther after mother's funeral and a series of fatastic incidents.
Koo was captivated by Star Wars film series when he was a child. After decades of actor careers in film industry, he expects the arise of Chinese own Sci-fi films, so he purchased several Visual effects and Prosthetic make-up companies. In 2013, Koo founded "One Cool Film Production Limited". As of 2017, One Cool Film has produced 14 films. The Sci-fi film Warriors of Future is on filming.
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). He also released Toys Battle (2008), a book chronicling his love for comic book characters and movie superheroes, and donated all proceeds to UNICEF. Koo has been writing blogs everyday on his own website since December 2006.
Koo recorded several theme songs for the TVB dramas he had starred in and several TV advertisement songs he had endorsed. He had also cut four albums: Boyfriend and Popular Louis in 2000, optimism in 2001, Mr.Cool in 2003.
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, Tag Heuer, Gillette, Epson printer, Hugo Boss Perfume, Clinique, Bally, Matisse, Johnnie Walker, Lipton, Lotte chewing gum, Lay's, Tropicana, Pentax camera, OSIM, Yokohama Tire, Toyota Corolla, Samsung Galaxy, Xbox 360 Kinect, DBS Black World MasterCard.
On 3 July 2012, Chinese microblogs caught on to the news of Koo's low profile philanthropy in China, which became a trending topic, more and more people born in different remote rural areas found that local elementary schools were built with donation of Koo, and upload photos to the microblogs. Major Chinese dailies further reported that he had helped build 49 schools, over 20 clinics and completed 50 water supply projects to help the underprivileged in remote areas in rural China for the past three years without any publicity. Koo was reluctant to comment on the issue but it was learnt that Koo set up his charity foundation after witnessing the devastation from the Sichuan earthquake in 2008. According to "Grace charity" website, Koo has helped build 97 elementary school buildings, 18 clinics and 751 small water supply projects in remote undeveloped areas of rural China through "Grace Charity Foundation Limited" and local government by 2017.
Koo is also a UNICEF (United Nations International Children's Emergency Fund) ambassador since 2007.