At the end, when Marco's recounted his whole crazy tale, he exclaims, "AND TO THINK THAT I SAW IT ON MULBERRY STREET!" This exclamation—which is also the title of the book—is a little silly and ironic, because Marco didn't actually see it all on Mulberry Street. All he saw was a horse and wagon, but in his mind he turned this mundane sighting into something big and wonderful and extraordinary.

Since this book is about the power of the childhood imagination, the title plays into the idea that Marco really believes in his imagination. He sees it as real, even when it's obviously just derived from a guy with a horse going down the street and minding his own business. Marco finds magic, even on plain old Mulberry Street, just by using his imagination.