[1] First published by Allison & Busby in 1974, she was best known for the Worst Witch novels and Large Family picture books, with sales amounting to several millions.

[6] She enjoyed reading boarding-school stories, which provided material and inspiration for Miss Cackle's Academy in the Worst Witch series, as did the Ursuline High School, Wimbledon, which she attended.

[9] Murphy started to write The Worst Witch – "the magical tale of an accident-prone girl attempting to navigate the magical codes and murky corridors of Miss Cackle's Academy for Witches"[8] – while still at school (Murphy admitted to having herself been the model for her protagonist Mildred Hubble),[10] but put the book on hold while she attended Chelsea and Croydon Art Schools.

[15] Murphy said she was "thrilled to find the publishers were quirky like me",[8] as she further revealed in an interview with The Telegraph: "They accepted it immediately, and printed 5,000 copies, and I remember wondering how many aunts and uncles I had, and what we would do with the rest".

[16] The book proved an instant success with both critics and readers (Murphy recalled that she "immediately got a lot of fan letters and the whole thing took off like a rocket"),[3][17] selling out within two months of publication in 1974.

[18] Murphy continued working as a nanny until Allison & Busby's publication in 1980 of The Worst Witch Strikes Again prompted her to devote herself to writing full-time.

A musical stage production called The Worst Witch Live, adapted by Emma Reeves from Murphy's original work,[21] was shown at the Royal & Derngate Theatre in Northampton as a Christmas show in 2018, and toured the UK in 2019 across some 16 different venues, including at the Vaudeville Theatre in London's West End from 24 July to 8 September 2019.