Being in agreement in all points with Octavius, it can help to understand the etymology of the abbreviation Mrs.... Take Our Word for it says " Mrs. is simply an abbreviation of the word mistress, which used to be pronounced "mistress" and was a title of courtesy. The contracted form of the word, pronounced "missus", became attached to the abbreviation, and using mistress (pronounced "mistress") as a title of courtesy fell out of disuse. It was most certainly by 1828 that the "missus" title was the only pronunciation of the word mistress and of its abbreviation Mrs. We find this quotation in that year, referring to mistress: "to pronounce the word as it is written would, in these cases, appear quaint and pedantick." Eventually the spelling mistress for "missus" fell out of disuse, and only Mrs. was left with that pronunciation, and it was only used as a title, preceding a name, as is still the case today.
Since we've mentioned mistress no longer being used in the above sense, we should explain how it came to mean "a woman who illicitly occupies the place of wife". It began as a snide metaphorical reference in the 15th century and simply stuck.
Mr. is simply the abbreviation of master, used as a title, like Mrs. The "mister" pronunciation arose simply as an alteration of master. Rather like its female counterparts, the "mister" pronunciation stuck with Mr., while master retained its "master" pronunciation, and the two were considered distinct words, this by the 18th century. Interestingly, the plural abbreviation is Messrs., taken from Messiurs as there was no extant plural form from the English word.
The ultimate origin of the above feminine forms is Old French maistresse.