Molly Lefebure was born in London, the daughter of Charles Hector Lefebure and Elizabeth Cox, and was educated at the North London Collegiate School. She went on to study at King's College London at the University of London where she met her husband, John Gerrish. During World War II she worked as a newspaper reporter for a London newspaper. During the war she met Dr Keith Simpson (the pathologist) and worked for him as his secretary. She gained information for her first book Evidence for the Crown, and went to live with her husband and her two children at Kingston-upon-Thames by the river.

Her children's books include illustrations by the famous Lakeland author, hill walker and illustrator Alfred Wainwright.

She is a Coleridge scholar. After studying drug addiction at Guy's Hospital in London for six years, she wrote a biography of Coleridge that researched the effect on his life of his addiction to opiates.

She was elected a Fellow of the Royal Society of Literature in 2010.