Marele Day was born and educated in Sydney, taking a BA in French and English from the University of Sydney. After teaching for several years, she travelled extensively, living in Britain, Europe, the USA, India, Nepal and Sri Lanka.
Day published her first novel, Shirley's Song, while overseas, but she is better known for the crime fiction written after her return to Sydney in 1987. Known as Claudia Valentine Thrillers, Day's crime novels subvert the male dominated crime novel by employing a female character as her private investigator. The series has been well received, and has been translated into German. During this period Day continued to support her writing with a fellowship and several grants from the Literature Board. In 1993 she won the prestigious Shamus Award for crime fiction, the first non-American to receive the award. In addition, she has written a guide to crime fiction, How to Write Crime (1996).
Day left the crime fiction genre with her last novel, Lambs of God (1997). Attracting many favourable reviews, the novel about an order of nuns whose solitary existence is under threat by developers, is also being produced as a film. The success of this novel has enabled her to curtail other work, such as editing, teaching and speaking while she works on other projects. Her most recent publication is the collection of short stories, Mavis Leavack PI (2000).
During her residency in Japan as a winner in the 2005 Asialink Writers' Residencies programme, Day researched the lives of female deep sea divers for her novel, The Sea Bed.