As it turned out, she wasn't fast enough to be more than a very good runner - until 1988. Aged 28 and at a stage in life when dramatic improvement should have been beyond her, she came blasting out of nowhere to win events at the United States Olympic trials and to set the first of her world records.
Astonishingly, her 12-year-old records still stand. For the trials, she wore a one-piece catsuit with one leg removed - exotic, thrilling, blindingly photogenic. It was a moment of genius in a life devoted to the limelight. For the 1988 Olympic Games in Seoul her nails were painted patriotic red, white and blue, with gold to symbolise her dream.
She won three gold medals - for the 100 metres (10.49 sec), 200 metres (21.34 sec) and 100 metres relay. More remarkably, she won a silver in the 400 metres relay - an event for which she hadn't even trained.
Her moment of victory in the 100 metres is one of those for-all-time sporting images - her zillion megawatt smile. Well, everyone smiles in victory - but not after 80 metres of a 100 metre race. She was so far ahead. She knew.
She expected to be taken to the bosom of America after her sensational victories, but America conspicuously failed to oblige. As Flo-Jo won in Seoul, so Be Johnson of Canada, winner of the men's 100 metres, was disqualified after testing positive for anabolic steroids - an event that changed sport forever.
The public lost its trust in track and field athletes. People certainly did not trust Flo-Jo. Everything, from her extraordinary leap of ability to her baritone voice, seemed to scream drug use. I asked her about it: "What do youtake, Florence? Is it essence of panther, or what?" She laughed - huskily - and said: "Ohhh nooo. I just gave it my ahhhhll."
She was an extraordinary creature - absurd in her self-obsession, a little shy in conversation, quite the opposite when the cameras came out. And she spoke about giving her "ahhhll" in her next projects - fashion design, nail-care products, motivational speeches, a children's book. And perhaps all is what she really gave, for she died suddenly aged 38, leaving a husband, Al Joyner, the 1984 Olympic triple-jump gold medal winner, and seven-year-old daughter, Mary Ruth. It was just 10 years since those few golden days in Seoul.
Everyone leapt to the conclusion that it was drugs that carried her off, but even in death the case remained unproven. The inquest said she'd suffered from a brain condition, had an epileptic fit in her sleep, and suffocated.
Flo-Jo was one of sport's willing victims - victim of the world's lust for heroes and heroines. She was one more sporting Icarus who flew too high and paid the price. At least we still have her records. Photograph by Tony Duffy
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Simon Barnes is a sports columnist for The Times