Molly Lefebure was born in Hackney on 6 October 1919 into a family descended from prominent arms manufacturers in 18th-century Paris. Her father, Charles Lefebure OBE 1941 Birthday Honours, was a senior civil servant who worked with Sir William Beveridge on the establishment of the NHS, applying some of the revolutionary ideas of Robespierre, the Parisian Lefebures having professed Jacobin sympathies. Some of Molly's forebears had been men of letters; and one, Pierre Lefebure, having helped to set up the Institut Francais, became a professor of languages at the newly formed London University  . Her uncle was Major Victor Lefebure (OBE, Chavalier of the Legion of Honour and Officer of the Crown of Italy) who, on 5–6 October 1916, carried out one of the most successful cylinder gas attacks of the war on the French front at Nieuport. He was a British Chemical Liaisons Officer officer with the French until the war closed. He wrote 'The Riddle of the Rhine: Chemical strategy in Peace and War' .
Molly Lefebure was born in London, the daughter of Charles Hector Lefebure OBE 1941 Birthday Honours 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 where she gained information for her first book Evidence for the Crown, the inspiration for the two part ITV drama Murder on the Homefront, which was also a title she coined for this memoir when she later republished it. Molly was the first woman ever to work in a mortuary. She was known as 'Molly of the morgue' and 'Miss Molly' by Scotland Yard.
She went on to live with her husband and her two children at Kingston-upon-Thames by the river. and also owned a house, Low High Snab, in Newlands Valley in Cumbria, where she wrote many of her books.
Molly's maternal grandmother arranged for her to spend summers on a remote farm on Exmoor, where Molly learned to hunt. Blooded aged eight with the Devon and Somerset Staghounds, she subsequently wrote on hunting for both The Field and Country Life and was a member of the Blencathra Hunt in the Lake District for more than 50 years.
Among her 20 or so other books was a 1974 biography of Coleridge, subtitled The Bondage of Opium, and a study of his wife, The Bondage of Love (1986), which won Molly Lefebure the Lakeland Book of the Year award. Her study of the Coleridge children, The Private Lives of the Ancient Mariner, is with her publisher, as is her last book, about the Lake District, The Vision and the Echo. She also wrote several novels, as well as (under the name Mary Blandy, an 18th-century forebear who was convicted of poisoning her father) two studies of drug addiction.
Her children's books include illustrations by the famous Lakeland author, hill walker and illustrator Alfred Wainwright.
She was 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.
- Evidence for the Crown: Experiences of a Pathologist's Secretary, London: W. Heinemann, 1955; Philadelphia and New York: J-B. Lippincott Company, 1954, 1955, also published as Murder on the Home Front: The Unique Wartime Memoirs of a Pathologist's Secretary, London etc.: Grafton Books, 1990, ISBN 0-586-20854-2
- Murder with a Difference: The Cases of Haigh and Christie, London: W. Heinemann, 1958
- The English Lake District, London: B.T.Batsford Ltd, 1964
- Scratch and Co – The Great Cat Expedition, with drawings by A. Wainwright, London: Victor Gollancz, 1968; New York: Meredith Press, 1969, ISBN 978-0-9547213-1-2; Blackburn: Mountainmere Research, 2006, ISBN 0-9547213-4
- Cumberland Heritage (Chapters include Camden, Braithwaite, Millbeck, Fellwalkers, Carlisle Canal, Armboth, John Peel (Farmer) and The Blencathra), with endpaper maps of old Cumberland, London: Gollancz, 1970, ISBN 0-575-00376-6
- The Hunting of Wilberforce Pike, illustrated by A. Wainwright, London: Victor Gollancz, 1970
- The Loona Balloona, Nashville, Tennessee: Thomas Nelson Inc., 1974, ISBN 0-8407-6378-6
- Samuel Taylor Coleridge: A Bondage of Opium, New York: Stein and Day, 1974, ISBN 0-8128-1711-7 and London: Victor Gollancz, 1974, ISBN 0-575-01731-7
- Cumbrian Discovery, London: Victor Gollancz, 1977, ISBN 0-575-02235-3
- The Bondage of Love – A Life of Mrs Samuel Taylor Coleridge; London: Victor Gollancz, 1986, ISBN 0-575-03871-3
- The Illustrated Lake Poets, 1987, ISBN 0-7112-0477-2
- Blitz!, London: Victor Gollancz, 1988, ISBN 0-575-04306-7, also published as We'll Meet Again, London etc.: Grafton Books, 1990
- Thunder In The Sky, London: Victor Gollancz, 1991 ISBN 0261669990
- Thomas Hardy's World: The Life, Times and Works of the Great Novelist and Poet, London: Carlton Books Ltd, 1997 ISBN 1-85868-245-2
- Private Lives of the Ancient Mariner: Coleridge and his Children, The Lutterworth Press (2013), ISBN 9780718893002.
- Oliver Gerrish (13 March 2013). "In memory of Molly Lefebure". Retrieved 18 April 2013.
- "Obituary: Molly Lefebure". Daily Telegraph. 18 April 2013. Retrieved 18 April 2013.
- Molly Lefebure, thePeerage.com.
- Cumberland Heritage.
- "Royal Society of Literature All Fellows". Royal Society of Literature. Retrieved 9 August 2010.