Earlier in her life she worked as an actress and model under the name Candy Davis and appeared as Miss Belfridge in the BBC sitcom Are You Being Served?

Born in Essex on 2 January 1962, Hayder grew up in Loughton as the daughter of John Bastin, an astrophysicist, and Susan Hollins (née Jacobsen), a teacher.

[4][1] After a brief marriage to Gary Olsen she emigrated at 25 to Japan[7][8] where she became a teacher of English as a foreign language in Tokyo.

To her surprise she was accepted by leading literary agent Jane Gregory who secured her an offer of nearly £200,000 from Transworld Publishers for a two-book deal.

[8] Her second novel, The Treatment, was a Sunday Times bestseller and won the 2002 WH Smith Thumping Good Read Award.

[11] Her fifth book, Ritual was the first in "The Walking Man" series, and was nominated for CWA Ian Fleming Steel Dagger award.

[16][17] Shortly before her death, she completed a new novel The Book of Sand, a speculative thriller written under the pseudonym Theo Clare, which was published in 2022.

[1] Hayder lived in Cheltenham, England with her second husband, Bob Randall, a retired police sergeant whom she married in 2021.