Born as Sybil Mary Edwards in Ramsey Heights, the daughter of a smallholder on the Fens who had left school at the age of nine, she was educated at Ramsey Heights Elementary School (1919–1923) and Ramsey Grammar School in Cambridgeshire (1924–1932). Marshall was not able to attend university because no scholarship was available so she started work in 1933 as an untrained teacher, first in Essex and then in Huntingdon. As an unqualified teacher at Kingston Primary School in Cambridgeshire from 1942 to 1948 she worked on her own in one room containing 26 pupils aged between 4 and 11. Here Marshall developed teaching methods based on integrating subjects and encouraging children's creativity. Later written up as An Experiment in Education, her methods influenced the 1967 Plowden Report into primary education in Britain. She attended Exhall Grange Emergency Training College in Coventry from 1948 to 1949, before going to Kingston County School in Cambridgeshire as Headteacher.
When this school closed because it was too small she went to New Hall, Cambridge in 1960 aged 48 to read English. She completed the three year degree course in two years. She was lecturer in primary education at the University of Sheffield from 1962 to 1967 and was an educational adviser to Granada Television for the series Picture Box from 1965 to 1978. She was Reader in Primary Education at the University of Sussex from 1967 until her retirement in 1976.
On retiring Marshall began a new career as a writer of fiction, writing her first novel at the age of 80 after a 10-year battle with cancer. Her trilogy – A Nest of Magpies (1993), Sharp Through The Hawthorn (1994) and Strip The Willow (1996) – are semi- autobiographical. She also published academic works on education and her childhood memoirs of growing up in the Cambridgeshire fenland. She was Sue Lawley's castaway on Desert Island Discs in 1993 and was a winner of the Angel Prize for Literature for her Everyman's Book Of English Folk Tales (1981). In 1995 she was awarded an honorary Doctor of Letters degree by the University of Sussex.
In 1939 she married Francis Marshall, a local farmer. She gave birth to a still-born son in 1940 and in 1941 had her daughter Prue, who later also became a headmistress. Frank Marshall found it hard to handle such an independently minded wife and left her for another woman. They divorced in 1948. In 1963 Marshall met the historian and illustrator Ewart Oakeshott at a dance. He left his wife for her and they became partners for life and married in 1995 at Ely Register Office, after the death of Oakeshott's first wife. Marshall was 82. Oakeshott predeceased her.
- "Obituary for Sybil Marshall". The Guardian. 31 August 2005. Retrieved 16 December 2017.
- "Sybil Marshall: Educationist and chronicler of Fenland life". The Independent. 4 September 2005. Retrieved 16 December 2017.
- "Marshall's Curriculum Vitae". Ramsey and District Cambridgeshire Community Archive Network. Retrieved 16 December 2017.
- Sybil Marshall on the BBC Radio 4 Desert Island Discs website
- Denis Hayes (2010). Encyclopedia of Primary Education. Routledge. p. 268. ISBN 0-415-48517-7 – via Google Books.
- University of Sussex Honorary Degrees Committee List of Honorary Graduates Retrieved 16 December 2017.
- "Sybil Marshall Obituary". The Daily Telegraph. 3 October 2005. Retrieved 16 December 2017.
- "Ewart Oakeshott". The Daily Telegraph. 12 October 2002. Retrieved 16 December 2017.
- Published by Pergamon Press in 1968 and illustrated by Ewart Oakeshott.
- All the Bibliographical detail taken from a paperback copy published by Penguin in 1995, the original being published in hardback by Michael Joseph in hardback on 1994 of Sharp Through The Hawthorn