You use mister before a man's name, usually the last name or the full name.
Mister Brakel, the doctor will see you now.

This is Mr. Terry McGinty.
You call a man mister when you don't know his name.
Excuse me, mister, I think you dropped this.
You use mister before a man's title for certain titles, such as justice, mayor, president, etc.
Yes, we can do that Mr. Mayor.

When you write it as part of somebody's name, you usually use Mr.

Noun