Mrs. Once Meant a Rich and Powerful Woman, Not a Wife
The title came from mistress, meaning a boss, just as Mr. came from master.

Mrs. used to mean a woman with money and power, not a wife. It was a shortened form of mistress, and mistress then meant a person in charge, the way someone runs a household, not a secret lover.
That is why marriage had nothing to do with it. As historian Stephanie Coontz explains it, a star like Taylor Swift would have become Mrs. Taylor Swift the moment she grew rich and famous, and stayed so for life, with or without a husband.
Mr. worked the same way, as a shortened form of master, meaning a person of wealth and standing. People below such figures were expected to manage their moods and problems politely, whether the boss was a man or a woman.
Questions people ask
When did Mrs. start meaning a married woman?
The marital meaning became standard in Britain and America in the 1700s and 1800s. As class titles faded, Mrs. narrowed to mean a married woman, while Miss was reserved for an unmarried woman or girl.
Where does Ms. fit in?
Ms. was promoted in the 20th century as a parallel to Mr., a title that does not reveal marital status. It spread widely in the 1970s, helped by Ms. magazine, and is now standard business usage.
Why is Mrs. pronounced "missis" if it is short for mistress?
Mrs. is a spoken contraction that kept the old pronunciation of mistress. The spelling shortened over time, but the "missis" sound stuck, which is why the letters and the sound do not seem to match.