The couple said 'I do' at a registry office in Marylebone on Sunday, in front of just 30 guests, including Beatles' bandmate Ringo Starr and Shevell's cousin, Barbara Walters.
American heiress Shevell, 51, wore a knee-length long-sleeved ivory dress designed by Paul's daughter Stella, and was given a 5-carat square-cut Neil Lane diamond ring by her new husband, according to Us Weekly.
An onlooker said that the newlyweds left The Old Marylebone Hall, which is also where McCartney married his first wife Linda Eastman in 1969, looking 'very much in love'.
'McCartney seemed over the moon. He was smiling and was holding hands with Nancy the whole time,' said the witness.
The couple held a reception at McCartney's home in St. Johns Wood, where he told the press, 'I feel wonderful.'
This is McCartney's third marriage after his first wife Linda died from cancer aged 56 in 1998 and his second marriage to Heather Mills ended in divorce.