Davis was born in Birmingham and after taking a degree in English literature at Oxford University (Lady Margaret Hall), she became a civil servant. She left the civil service after 13 years, and when a romantic novel she had written was runner up for the 1985 Georgette Heyer Historical Novel Prize, she decided to become a writer, writing at first romantic serials for the UK women's magazine Woman's Realm.
Her interest in history and archaeology led to her writing a historical novel about Vespasian and his lover Antonia Caenis (The Course of Honour), for which she couldn't find a publisher. She tried again, and her first novel featuring the Roman "detective", Marcus Didius Falco, The Silver Pigs, set in the same time period and published in 1989, was the start of her runaway success as a writer of historical whodunnits. A further eighteen Falco novels have followed, as well as The Course of Honour, which was finally published in 1998. Rebels and Traitors, set in the period of the English Civil War, was published in September 2009. Nemesis, the twentieth Falco title, is to be published in June 2010, as is The Official Companion. Davis has won many literary awards, and was honorary president of the Classical Association from 1997 to 1998.
The dedication of her 2009 work Rebels and Traitors is "For Richard / dearest and closest of friends / your favourite book / in memory", and the author's website tells that "I am still getting used to life without my dear Richard. For those of you who haven't seen this before, he died in October ". The author says in her publisher's newsletter: "The greatest recommendation I cangive is that Richard, its first reader, thought it wonderful. He devoured chunks, demanding ‘Bringmore story!’ even when he was in hospital. One of the last things I was ever able to tell him was that Rebels and Traitors was to be published by Random House, so I would be working with dear friends for his favourite book."