The couple was greeted by fans as they arrived at the registry office at about 3:30 p.m. and posed for photographs before entering Old Marylebone Town Hall, MTV News reported.
Following the service, a reception was held in the star's garden at his home in St. John's Wood.
The ceremony itself was low key and not widely publicized.
The guest list of about 30 people included Ringo Starr and Pink Floyd's David Gilmour. McCartney's daughter, Stella, designed her new stepmom's dress and helped put together the organic menu for the reception, the BBC reported
The Mirror said the couple selected Sunday as the wedding date because it was John Lennon's birthday.
Paul and Nancy were seen out on the town Saturday at a popular Chinese restaurant in London's Regent's Park.