She was born Florence Harriet Montgomery in Chelsea, London, on 17 January 1843, the second of the seven surviving children of Admiral Alexander Leslie Montgomery (1807-1888) and his wife Caroline Rose Campbell (1818-1909) of Hampton Court, Middlesex.
Florence Montgomery's story-telling abilities were first tried on younger brothers and sisters, but the novelist G. J. Whyte-Melville saw a story of hers printed for a charity bazaar, about a golden-haired girl whose mother dies of scarlet fever, and advised that it should be published.