The 53-year-old mother-of-two bought the ticket at a Chicopee, Massachusetts, petrol station.
Mavis Wanczyk told reporters she had already quit her job.
The lucky numbers were 6, 7, 16, 23 and 26, and 4.
The biggest ever US Powerball jackpot was $1.6 billion, although that was shared by three ticket holders in January 2016.
Massachusetts State Lottery told reporters the ticket in August 23 draw had been validated.
Mavis Wanczyk told reporters: “The first thing I want to do is just sit back and relax.
“I had a pipe dream.. and it came true.”
She said she had picked her lucky numbers based on relatives’ birthdays.
Mavis Wanczyk said of her 32-year job at a medical center: “I’ve called them and told them I will not be going back.”
She added that she was “going to go hide in my bed”.
Reporters asked if she had plans to treat herself to something nice, such as a fancy new car.
Mavis Wanczyk replied she just bought a new car less than a year ago, and now plans to pay it off in full.
One lottery officials described her as “your prototypical Massachusetts resident”.
He added that Mavis Wanczyk seems like “a hard-working individual” and “clearly she’s excited”.
The $50,000 prize awarded to the business that sells the winning numbers will be donated to charity, said Pride petrol station owner Bob Bolbuc.
The jackpot payout, which can be made in 29 yearly payments or a lump sum, is estimated to be about $443 million.
Powerball Product Group chairman Charlie McIntyre said in a statement that six other tickets – sold in Connecticut, Illinois, Louisiana, New Mexico, Pennsylvania and the Virgin Islands – won $2 million each.
Another 34 tickets across the US scooped $1 million.
Massachusetts lottery officials initially said the jackpot ticket was sold at a convenience store in the Boston suburb of Watertown, but corrected the location on August 24.
It is not clear how the error was made.
Odds of winning the jackpot are one in 292.2 million.
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