"Nancy and Paul are getting married," a source close to Shevell, 51, a New York businesswoman, first told PEOPLE Friday. "Ring and all – very exciting."
McCartney's rep later confirmed, but would not provide any more details. The Beatles legend, 68, started dating Shevell in the Hamptons during the summer of 2007, and despite their 17-year age difference, a source close to the couple told PEOPLE they were a perfect pair.
"They have the right chemistry," the source said. "They're both cool, chilled out and optimistic."
The couple most recently stepped out on Monday for a gala at the Metropolitan Museum of Art in New York City.
In LoveThis will be McCartney's third marriage: He married Linda McCartney in 1969 and Heather Mills in 2002. While his marriage with Mills ended in a messy divorce in 2008, McCartney didn't seem hesitant to give love another shot.
"I just like being in love," he said of his budding romance with Shevell in 2008.
His fiancée even won over the kids – McCartney has five children: Heather, 48, Mary, 41, Stella, 39, James, 33, and Beatrice, 7 – and, in 2009, a source close to Stella said the fashion designer was "thrilled for her father."
With reporting by GAIL NUSSBAUM