Day was born in Sydney, and grew up in Pagewood, an industrial suburb. She attended Sydney Girls High School and Sydney Teachers' College and in 1973 obtained a degree from Sydney University. She has worked as a patent searcher and as a researcher and has also taught in elementary school during the 1980s.

Her Claudia Valentine series features a feminist Sydney-based private investigator but her breakthrough novel was Lambs of God which was a departure from the crime genre and features two nuns battling to save the island on which they live from developers; it became a bestseller.

She lives on the New South Wales North coast.

Marele Day's four book Claudia Valentine series has become a minor classic in Australian crime writing, but her Lambs of God (1998) was even more highly acclaimed as an original and provocative literary work, published in the US by Riverhead and in the UK by Sceptre. Her most recent novel was Mrs Cook, a rich portrayal of the life of a woman whose passion and intellect matched that of her celebrated husband.