She was the fifth child of St John Hornby, a founding partner of W. H. Smith and a deputy vice-chairman of the NSPCC, and his wife, Cecily.
In 1942, Rosamund Hornby joined the Women's Voluntary Services, giving up her job as assistant matron of the girls' school which had been evacuated in 1939 to Chantmarle, the family home in Dorset.