Mr is correct or Mr. is correct?

Answer 1

In the United States, we use a period. In the UK, one does not. I have no idea why this is so.

An explanation:

In British English, a full stop replaces one or more letters that have been omitted from a word. 'Mr' represents the first and last letters of the word 'Mister'. No letters have been omitted after the 'r', so there is no full stop. Similarly there is no full stop after 'Dr' (Doctor) or 'Mrs' (Mistress, popularly altered to Missus). No abbreviation that includes the last letter of the original word should be followed by a full stop. To British eyes, that shows ignorance.

Examples of the correct (to British English speakers) use of a final full stop include 'e.g.' (short for 'exempli gratia') and 'i.e.' (short for 'id est'). However, there is a growing tendency to leave out final full stops in abbreviations generally.