I just realized I never hear works set in medieval times use "Mrs.", it's always Lady So-and-So or some nameless peasant.
Well, Mrs. is a contraction for "mistress" (the feminine version of "mister" and was originally used for both married and I married women.
As for earliest recorded use, I'm unsure. The farthest back I could find was in Shakespeare's The Merry Wives of Windsor, but I'm sure there are references before that.