Jill was born and brought up in London and says that she can't remember a time she wasn't a storyteller/illustrator. By the age of six, Jill was stapling her own little illustrated storybooks together - her mum kept them all and she still has them today! Jill left school at sixteen, and went on to attend both Chelsea Art School and Croydon Art School. Two years later, she finished her first novel The Worst Witch, about a little witch who doesn't fit in at her new school - a story she says was heavily based on her experiences at the convent. She sent it off to three big London publishers, who all turned it down, and Jill "put it in a drawer and decided to concentrate on other things instead." After a spell working as a nanny and in a children's home, which she loved, Jill had a phone call from a small publisher interested in The Worst Witch. It was printed when she was twenty-four, sold out almost immediately, and Jill knew for sure she wanted to follow a career as an author/illustrator.
Jill has written and illustrated numerous books since The Worst Witch was published, including three more books about little witch Mildred Hubble. Jill's work has won her numerous prizes and awards; her first picture book, Peace at Last was Commended for the Kate Greenaway Medal and she was shortlisted for the same Medal for A Quiet Night In, the first of the Mr and Mrs Large picture books about the endearing domestic chaos of an elephant family. The Last Noo-noo, about a small monster called Marlon, won three awards, including the Smarties Book Prize, and Marlon returns again in the acclaimed All For One.