Mok attended Diocesan Girls' School from primary to secondary grade in Hong Kong. She later left Hong Kong in 1987 and studied abroad. She attended United World College of the Adriatic near Trieste, Italy, and majored in Italian literature when studying in the University of London. Mok is the sister of the writer and producer Trevor Morris and the descendant of Alfred Morris, the first principal of King's College, Hong Kong. She speaks , , , , . She is Eurasian as her mother is quarter-Persian, quarter-German on her father's side, half-Chinese from her mother's; father is half-Welsh on his father's side, half-Chinese from his mother's.
Mok is often credited as Karen Mok in Chinese movies, but as Karen Joy Morris in Hollywood movies. She supplied the voice of Princess Kida for the Cantonese dub of Disney's "" . Mok performed in the international tour of the hit Broadway musical "" as Mimi during the Hong Kong stops in December 2005.
Mok had her first high-profile relationship with actor-director Stephen Chow, after meeting at a film's production set back in 1994. Shortly, they were seeing each other. However, they then broke up due to different values.
Subsequently, in 1998, she was romantically linked with actor-singer-director Stephen Fung. And from then on, their relationship blossomed, till the break-up in early 2008. Many were shocked when Mok announced this news during her L!VE is… KAREN MOK's mini-concert. And she insisted that there was no third party between them, as the media speculated that Stephen was the one who gave up their 9-years relationship for another woman.