She was born Violet Mary Firth in Llandudno in 1890, worked as a lay psychotherapist in London before her occult career, and took her magical name from her family motto, Deo, non fortuna. The psychological training is the key to her.
Fortune's achievement was expository. The Golden Dawn material had been secret, then leaked, and was written in a private idiom that assumed its own cosmology. She restated it, most durably in The Mystical Qabalah (1935), as a psychology of the whole personality, in ordinary English, for readers who had not been initiated into anything. Psychic Self-Defence (1930) did the same for a subject that had previously been either sensational or silent. She founded the Fraternity of the Inner Light, later the Society of the Inner Light, in the 1920s.
Her occult novels, The Sea Priestess and Moon Magic in particular, carried a doctrine of magical polarity between the sexes that has aged into her most contested legacy; it is essentialist in a way most later practitioners have had to rework or discard, and it should be read as of its period rather than quietly modernised.
The claim she made about her own experience, a psychic attack in 1911 that left her incapacitated for years, at the hands of a former employer, is her account and there is no independent test of it. It is also the origin of her most useful book, which is a fair illustration of how this tradition works: an unverifiable private experience produces a body of practical instruction that thousands of people have then found usable.