Agreeing with the answer selected, but also with the caution expressed by Frédéric Guéreau that, when addressing the person, we sometimes say, "Mister Mayor".
I once worked in the City Hall of a midsized US city, reporting to the City Manager, but officing adjacent to the Mayor's office. We usually referred to him as "the Mayor" or "Mayor [family name]" but occasionally as "Mister Mayor" when speaking to him, sometimes even as "Mister Mayor" when not in his presence...
It's pretty similar to "Mr. President", often used by reporters and others who are not well acquainted with the title holder personally. At the level of local government, there might be more of a presumption that everyone is on a more friendly basis of acquaintance.