McCartney, 69, and Shevell, 51, have been dating for four years and both are coming off divorces.
Rumors about their plans to marry began circulating last week, but nothing was official until they appeared at Old Marylebone Town Hall, where fans and paparazzi were waiting.
Access Hollywood and other sites were reporting as late as Friday (Oct. 7) that the couple would not marry this weekend.
Shevell wore an elegant, above-the-knee ivory dress designed by McCartney’s daughter Stella.
McCartney looked more like a banker than a rock star. He wore a dark blue suit with a white shirt and a solid, sky-blue silk tie.
For McCartney, his third marriage hopefully will be the charm.
He married first wife, Linda Eastman, at the town hall in 1969, the year before the Beatles broke up.
They remained happily married and had four children, until she died in 1998 from breast cancer.
He married his second wife Heather Mills in a lavish ceremony at Ireland’s Castle Leslie in 2002. More than 300 people attended.
The marriage, however, ended in a messy divorce in 2008. They had one child together.
Shevell, the daughter of a successful New Jersey business man, met McCartney in the Hamptons, where they were neighbors for 20 years.
They went public with their relationship in Nov. 2007. At the time, Shevell was separated from her husband of 20 years, attorney Bruce Blakeman.
The couple got engaged in June.