Certain eccentricities began when she and her husband were posted to Istanbul where he was the British consul. Her Letters from Turkey are said to be a model of perceptive writing about the exotic east. Her quirks sound like what we today would probably call an independent spirit and ability to think for herself. For instance her friendship with Sir James Steuart, an aristocrat, but also a Jacobite, was one of mutual admiration. Her habit of wearing harem trousers seemed to have completely unsettled Horace Walpole. Lady Montagu spent a lot of time on the continent; travel was a kind of 18th century internet for those of wealth.
Her correspondence would later add to her fame, and we quote from some with Sir Steuart and his wife Lady Fanny. (The Letters of Lady Mary Wortley Montagu, 1876). The letter dates to 1758, and in it she sketches a fantasy about herself being a witch. Of course it had been generations since anyone in England took that seriously. But she was inspired by a book she had been reading.
...[W]hat these prodigies portend, God knows; but I never should have suspected half the wonders I see before my eyes, and am convinced of the necessity of the repeal of the witch act (as it is commonly called), I mean, to speak correctly, the tacit permission given to witches, so scandalous to all good Christians: though I tremble to think of it for my own interests. It is certain the British islands have always been strangely addicted to this diabolical intercourse, of which I dare swear you know many instances; but since this public encouragement given to it, I am afraid there will not be an old woman in the nation entirely free from suspicion. The devil rages more powerfully than ever: you will believe me when I assure you the great and learned English minister is turned Methodist, .... and I have been seen flying in the air in the figure of Julian Cox,[discussed in the book] whose history is related with so much candor and truth by the pious pen of Joseph Glanville, chaplain to King Charles. I know you young rakes make a jest of all those things, but I think no good lady can doubt of a relation so well attested. She was about seventy years old (very near my age), and the whole sworn to before Judge Archer, 1663: very well worth reading, but rather too long for a letter. You know (wretch that I am) 'tis one of my wicked maxims to make the best of a bad bargain; and I have said publicly that every period of life has its privileges, and that even the most despicable creatures alive may find some pleasures. Now observe this comment; who are the most despicable creatures? Certainly, old women. What pleasure can old woman take? Only witchcraft. I think this argument as clear as any of the devout Bishop of Cloyne's metaphysics [a reference to George Berkeley's obscure writing]; this being decided in a full congregation of saints, only such atheists as you and Lady Fanny can deny it. I own all the facts, as many witches have done before me, and go every night in a public manner astride upon a black cat to a meeting where you are suspected to appear: this last article is not sworn to, it being doubtful in what manner our clandestine midnight correspondence is carried on. Some think it treasonable, others lewd (don't tell Lady Fanny); but all agree there was something very odd and unaccountable in such sudden likings. I confess, as I said before, it is witchcraft. You won't wonder I do not sign (notwithstanding all my impudence) such dangerous truths: who knows the consequence? The devil is said to desert his votaries.
Yes there is some context missing in the above. It is still funny.