She is 1/2 Chinese, 1/4 Welsh (paternal grandfather), 1/8 Iranian and 1/8 German. Mok attended Diocesan Girls' School, Hong Kong (from primary to secondary), majored in Italian literature when studying in the University of London and United World College in Trieste, Italy. She 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. Mok speaks English, Cantonese, Mandarin, Italian, French.

Mok is often credited as Karen Mok in Chinese movies, but as Karen Joy Morris (her birth name) in Hollywood movies. She supplied the voice of Princess Kida for the Cantonese dub of Disney's Atlantis (2001). Mok performed in the international tour of the hit Broadway musical "RENT" as Mimi during the Hong Kong stops in December 2005.