Mistress has 8 letters; besides, Anne of Cleves was never the King's mistress; the moment he clapped eyes on her he could hardly bear to touch her, which cost poor Thomas Cromwell his head.
Both were from "noble" families, for want of a better term, so never had professions in the traditional sense. As has been stated, Jane Seymour was a maid-of-honour, but I don't believe Anne of Cleves was.