Geras' father was in the Colonial Service and so had a varied childhood living in countries such as Nigeria, Cyprus, Tanzania Gambia and British North Borneo in a short space of time.She attended Roedean School in Brighton and then graduated from St. Hilda's College, Oxford University with a degree in Modern Languages. She was known for her stage and vocal talents, but decided not to pursue that career, instead became a full-time author.

Geras's first book was Tea at Mrs Manderby's which was published in 1976. Her first full length novel was The Girls in the Velvet Frame. She has written more than 95 books for children, young adults and adults. Her best-known books are Troy ( shortlisted for the Whitbread Prize and Highly commended for the Carnegie Medal) Ithaka, Happy Ever After (previously published as the Egerton Hall Trilogy) and Silent Snow, Secret Snow.As well as"A Thousand Yards of Sea".Her novels for adults are:Facing the Light,Hester's Story,Made in Heaven,and A Hidden Life.

Geras won two prizes in the U.S., one the Sydney Taylor Book Award for the My Grandmother's Stories and the National Jewish Book Award for Golden Windows. She's also won prizes for her adult poetry and was a joint winner of the Smith Doorstop Poetry Pamphlet Award, offered by the publisher of that name.