Should the word 'mister' be capitalized when it is used in the middle of a sentence?

Yes, the Word "mister" should be capitalized when it is used in the middle of a sentence if it is followed by a proper name, like "Mr. Smith." It's a sign of respect, even if it feels like a little extra work. So, show some class and hit that shift key when you're talking about Mr. So-and-So.