I'd be curious too. I'm still kind of new to the MBTI types, but I think he's an introvert. A lot of his songs are about not fitting in (e.g. "Have you ever been alone in a crowded room?" from Dark Blue and "And there was never any place for someone like me to be totally happy" from Cavanaugh Park).
I think he's probably perceiving because the whole speech in "I'm Ready" ("When did society decide that we have to change
Or wash a T-shirt after every individual use?") seems to fit that.
He doesn't really come across as an INFP to me though (which I am). So I'm not sure. Maybe an ISFP?
Die-hard Andrew McMahon fan. I'm fairly certain he's an extrovert that uses the introverted feeling function (Fi) as his secondary. As an INFP I resonate strongly with his expressions. He's very in tune with the self and his own values/the nuances of his feeling, and most of his songs have that kind of flavor, which is much more of an Fi than Fe trait.
My guess would be ENFP. He's very introspective, but also very at ease in crowds and very physically expressive. In my experience, ENFPs often have some characteristics that look like introverts. The conceptual content of his songs makes me lean towards intuitive like the rest of you all, rather than ESFP Also, he seems to use Si... his concerts often have certain patterns/traditions in them, and he throws in content that has a strong sentimentality to it designed to appeal to his longtime followers.