The 54-year-old Grasso said she was shopping at a grocery store near Fenway Park last Tuesday when she noticed a crowd of people buying tickets.
"...I saw 'Big Prize 197 million' and I said, 'I'll take a chance,'" she told a press conference Wednesday.
Grasso, who became a U.S. citizen in 1984, takes care of four children for Chris Gabrieli, a millionaire venture capitalist who made an unsuccessful bid for Congress last year.
When she heard the winning numbers on the news, Grasso said she couldn't believe she had won. It was only when she checked the numbers again that she realized her good fortune.
With the winning ticket beside her bed, she had difficulty sleeping.
"At first I couldn't believe that it could be. It can't be me. There are so many million chances," Grasso said.
Grasso was born in Chile, where her family still lives. She was divorced in the mid-1980s and moved from New York to Boston in 1996. She has two children in college.
For the last three years, she has been caring for Gabrieli's children, who range in age from 1 to 5.
She said she has no clear plans for the money, but that she is not anxious about her huge windfall.
"I'm a very down-to-earth person. I've worked hard all my life," she said.
Her work has included stints as a teaching assistant with the mentally handicapped.
"This has been my priority, to work with children, special needs children. I think this will be a chance for me to keep helping," she said.
However, Grasso said she's not planning to work again any time soon, having landed the largest lottery prize ever won by an individual in U.S. history. Grasso said she would take the money in one lump sum. She expects to net about $70 million after taxes, her lawyer said.
Grasso said she chose the winning combination -- 12, 17, 22, 33, and 44 with the Big Money Ball of 25 -- at random. The odds of winning were 76 million-to-1.
Wednesday's announcement finally put to rest rumors that the winner was a Nigerian immigrant cab driver.
The rumor mill started with Town Taxi, where drivers were reporting that cabbie Patrick Okusanya had won the huge jackpot. The speculation was further fueled when the driver didn't return to work after the Tuesday drawing.
Okusanya sought the help of an attorney to deny he was the winner.
"I felt sorry for him," Grasso said.
Starting in February, 328 million tickets were sold in the six Big Game states Massachusetts, Georgia, Illinois, Maryland, Michigan and Virginia, with no one winning the twice-a-week drawing until April 6.