Moriarty was raised in the northern suburbs of Sydney. She has five sisters and one brother. Two of her sisters, Liane and Nicola, are also novelists. Moriarty studied English at the University of Sydney upon graduating from high school. She then studied law at Yale University and Gonville and Caius College, Cambridge, where she was awarded a PhD. Moriarty also has a B.A. and an M.A.

She worked as an entertainment and media lawyer for four years before becoming a full-time writer. Moriarty was previously married to Canadian writer Colin McAdam, and they have one young son, Charlie. She currently lives in Sydney.

The Ashbury/Brookfield Series is four novels that are not sequels but are linked. They all revolve around various students that attend the exclusive private school, Ashbury High, or the local comprehensive, Brookfield High. Many of the students cross over into more than one novel, but each novel is different and tells a different story. All novels are told through the various character's own writing (through letters, emails, exam papers, etc).

The Ashbury/Brookfield series of novels are (in chronological order):

• Feeling Sorry for Celia
• Finding Cassie Crazy (AUS/UK title) The Year of Secret Assignments (US title)
• The Betrayal of Bindy Mackenzie (AUS title) also published as Becoming Bindy Mackenzie (UK title), and The Murder of Bindy Mackenzie (US title)
• Dreaming of Amelia (AUS/UK title) also published as The Ghosts of Ashbury High (US title)