The president of the United States was not always called “Mr. President”.
Washington wanted citizens to call him “His Mightiness, the President,” but to many people, that sounded too much like addressing the king they’d fought so hard to shed.
The next president, John Adams, thought he should be called “His Highness, the President of the United States and Protector of their Liberties,” but even fewer people went along with that.
Finally, Thomas Jefferson began shedding the trappings of imperialism that earlier presidents had insisted on.
After his inaugural, he walked back to his boarding house instead of riding in a coach.
He was also the first president to actually shake hands with ordinary people, previous presidents had greeted people only with a slight, dignified bow.
People began calling him “Mr. President,” a good mix of respect and equality.