Johnson, 28, has been in the wilderness for two years, suffering with a series of injuries, and lost her lottery funding at the end of last season.
But she confirmed her Beijing spot by winning the Olympic trials, a month after achieving the qualifying mark with a personal best of 6.81m at the European Cup.
"I always knew I'd be back," said Johnson. "I am a big stage person and for me, it's always been about the Olympics. I was getting down but I'm a fighter."
Long jump qualifying is on 19 August, with the final three days later, at 1220 BST.
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