Mayor Prentiss is Big Brother. He's the dictator who brainwashes his townspeople (you know, after killing off the real Mayor), and his big idea is his "I AM THE CIRCLE AND THE CIRCLE IS ME" campaign. Yes, it has to be in all caps. It's all part of the propaganda.
Todd explains that he "believes, see, that order can be brought to Noise" (2.52). But nice as this idea sounds, this "order" is actually control and manipulation. By bringing everyone into his circle, Mayor Prentiss can control the world. Using Noise, he beats this message into the people's heads, and rather than making people feel safe, it's eerie. When Todd's walking past the Mayor's house at the beginning of the book he describes: "His Noise is awful clear and I mean awful in the awful way" (2.52). That's a pretty clear message about who this boss dude is.
We don't meet him the Mayor too often in the story. He doesn't have a personality, like our other villain, Aaron, and he definitely isn't crazy (like Aaron). He's more of an idea, a force, a figurehead of all things evil. Kind of like Voldemort in Harry Potter—only without the missing nose.