Mr. or Mr?

The American usage is to place a period after Mr., Mrs., Ms. and Dr.; the British do not (Mr  Mrs  Ms Dr).

Both British and American usage require a period if the abbreviation is a truncation - i.e.,  if it does not contain the last letter of the word (the r in Mr is the last letter of the word mister): e.g., Prof. Pres. Hon. Capt. Gen.