Sir Paul McCartney and Nancy Shevell have filed their intention to marry at Old Marylebone Town Hall in London.
No wedding date has been announced, but it cannot happen for at least 16 days. Mrs Cathcart is expected to conduct the ceremony, which is expected to take place in October.
"It was all a bit last-minute," she said. "I had no prior warning. They were lovely, very relaxed and very charming and clearly in love with each other. They were excited."
Mrs Cathcart has been Westminster City Council’s superintendent registrar for almost 20 years, and has previously married celebrities including Sylvester Stallone, Joan Collins and Barbara Windsor.
"We have been called the Rock 'N' Roll Venue, and that’s a legacy of The Beatles because also Ringo Starr got married [to Barbara Bach] here. People like Liam Gallagher chose to get married here because of the Beatles connection.
"It's such an iconic building, and part of that is the knowledge that Paul McCartney chose to marry [Linda] here, and to think that he's doing it again is just incredible."
The precise location of the wedding is not known. The registry office contains four rooms, ranging from one seating 20 to the largest which holds 100.
McCartney, 69, and Shevell, 51, have been together since 2007. The wedding will be his third and her second. McCartney's first wife Linda died of breast cancer in April 1998, and his second marriage, to Heather Mills, lasted from 2002-2008.
"The best thing about being a registrar is the knowledge that in one moment you have changed people's lives, and I'm delighted that Paul and Nancy have chosen Marylebone Town Hall for their special day," Mrs Cathcart added.
"I am honoured to be conducting the ceremony and will get to see the look of joy on both their faces when I declare them husband and wife."