The singer and his wife Nancy Shevell were in the city for Sunday's (03Feb13) Super Bowl sporting event, and MCCartney hopped on a street car during his trip.
Local resident Robert Senft claims the music legend began singing Fab Four songs - but it took a while for passengers to realise who was performing.
He tells Britain's Daily Mirror, "This guy got on the street car and started singing Beatles songs. Everyone was just ignoring him because there's a lot of buskers in this town.
"He started to sing directly to my friend Evelyn. We realised it was Sir Paul MCCartney and were in shock. He started singing with the whole carriage and taking pictures. It was a once in a lifetime experience."