Murphy showed an interest in writing and drawing at the age of 6, and despite not excelling in other school subjects, had made her own enormous library of hand-written and illustrated books while still at primary school. She attended the Ursuline High School, Wimbledon which, together with the boarding school stories she enjoyed reading, provided much of the material and inspiration for Miss Cackle's Academy in The Worst Witch series. She grew up a Roman Catholic, but is no longer practising. Her stay-at-home mother was a "book maniac" and her father was an Irish engineer.

She started to write The Worst Witch while still at school, but put the book on hold while she attended Chelsea and Croydon Art Schools. She continued to write it during a year living in a village in Togo, West Africa and later while working as a nanny back in the UK. The book was published when Jill was 24 and proved an instant success. Jill continued working as a nanny until the publication of The Worst Witch Strikes Again prompted her to devote herself to writing full-time.

In 1986, a television movie with the same title as her fantasy novel premiered on HBO. It later aired on The Disney Channel during the 1990s around the time of Halloween.

She gave birth to her son Charlie in spring 1990 at age forty.

In 1995, she was diagnosed with breast cancer, but made a full recovery.

Both Jill's aunt and her mother had dementia, one after the other, during a period of seven years. It was difficult for her to work during this time, hence the reduction in her output between 1997 and 2004.

The Worst Witch stories have become some of the most successful titles on the Young Puffin paperback list and have sold more than 3 million copies. They were also made into a successful ITV series.

She is also known for her picture books, in particular the popular Large Family picture books, which detail the domestic chaos of an elephant family. The "Large Family" is now a TV series on CBeebies and ABC Kids. In 1996 The Last Noo-Noo was adapted as a play and performed at the Polka Theatre, London.

In 2007, Jill received an honorary fellowship from University College Falmouth.

The Worst Witch

• 1. The Worst Witch (1974)
• 2. The Worst Witch Strikes Again (1980)
• 3. A Bad Spell for the Worst Witch (1982)
• 4. The Worst Witch All at Sea (1993)
• 5. The Worst Witch Saves the Day (2005)
• 6. The Worst Witch to the Rescue (2007)

The Large Family (Picture Books)

• Five Minutes' Peace (1986)
• All in One Piece (1987)
• A Piece of Cake (1989)
• A Quiet Night in (1993)
• Mr. Large in Charge (2005)
• Laura Bakes a Cake (2008)
• Luke Tidies Up (2008)
• Lester Learns a Lesson (2008)
• Lucy Meets Mr Chilly (2008)
• Grandpa In Trouble (2009)
• Sebastian's Sleepover (2009)

Non- Series Works (Picture Books)

• My Teddy (1973)
• On the Way Home (1982)
• Whatever Next! (1983)
• Peace at Last (1980)
• What Next, Baby Bear! (1984)
• The Christmas Babies (1992)
• The Last Noo-noo (1995)
• All Aboard (1996)
• All for One (2002)

Non- Series Works (Novels)

• Geoffrey Strangeways (1985)
• Worlds Apart (1988)
• Jeffrey Strangeways (1992)