Rhiannon Lassiter was born on the 9th of February in 1977 in London to children's books author, Mary Hoffman and Stephen Barber.

She started writing the first book of the Hex trilogy when she was seventeen, and sent the first chapters of the MS to Douglas Hill (a friend of the family) and Pat White (her mother's agent). She was stunned when Pat wrote back saying that she loved it and would like to represent Rhiannon and Douglas said she should send it to his editor, Marion Lloyd, at Macmillan. Macmillan accepted the first two Hex books shortly after her nineteenth birthday.

As well as writing she also runs her own web-design business, writes articles and reviews of children's books and is part of the production team of Armadillo, her mother's children's books review publication.