Recently I read in the New York Times about a misuse of campaign funds investigation in New York state. Among other peccadilios to be investigated, one involved a legislator who was alleged to use campaign funds to pay for a girlfriend's apartment and visits to tanning parlors.
In my opinion, paying for her apartment and tanning puts her in the mistress category.
But wait! Maybe this is just a sign of the times; or a more sophisticated usage. Or a way of fending off eager lawyers. Has the term girlfriend expanded into a euphemism? I've already commented on the vagueness of that term as including female pals or buddies as well as women that a person (male or female) has an emotional relationship with. But this is a different category of girlfriend: one that a man does not bring home to meet one's mother. But maybe one's dissolute uncle!
This gives a new twist to that horticulturally-inclined Mistress Mary.