A reader asks Michael Quinion at World Wide Words where the r in Mrs comes from. If I were asked the same question, I’d probably guess it’s from mistress but wouldn’t be able to elaborate more from that point on. As always, Quinion provides deeper, interesting information:

The word mistress is a shadow of what it once was. It came into English from Old French in the fourteenth century as the female equivalent of master, a woman who has authority or who exerts control of some sort over other people. It usually referred to the female head of a household. It was common in later centuries to call a husband and wife “the master and mistress”. And mistress was also a title of respect conferred on the wives of farmers, the lower clergy, small tradesmen and the like — recall Shakespeare’s Mistress Quickly, an innkeeper in four of his plays.

In writing, mistress was conventionally shortened to forms such as Mres or Mris. The churchwarden’s accounts of St Mary at Hill in London recorded in 1485 a gift of a pyx cloth from “Mres. Sucklyng”. From the seventeenth century, the abbreviation was limited to the title mistress when it was attached to a proper name. The Mrs spelling had already begun to appear, in the later sixteenth century, initially in accounts, church records and the like. 

Make sure to read the whole entry, as Quinion also explains why Mrs is pronounced missis or missus, as per the same reader’s request.