Mary was born in 1727, the fifth and youngest daughter of John Campbell, Duke of Argyll, and his second wife, Jane who was a maid of honour to Queen Anne and Caroline. She married Edward, Viscount Coke, in 1747. However, the marriage was strained from the start, and before long was the matter of court proceedings. She moved to live with her mother at Sudbrook, Surrey; but, in any case, her husband died shortly after, when she was only 26.
With a legacy from her father, Lady Mary was able to play a full, if increasingly odd, part in society, travelling frequently in Europe. In the early 1770s, she became involved with the court of Empress Maria Theresa in Vienna; but, on a third visit, she fell out of favour with the Empress, and thereafter felt persecuted by her. Lady Mary had a long friendship with Horace Walpole, who dedicated The Castle of Otranto to her; but he called her ‘violent, absurd and mad’ (according to Ponsonby - see below). She died on 30 September 1811. Wikipedia has further biographical information.
Much of what we know of Lady Mary today comes from her voluminous diary, largely written in the form of letters to her sister Lady Strafford. This was first published privately (only 100 copies were printed) between 1889 and 1896 in four volumes by David Douglas in Edinburgh. These were reprinted in 1970 by Kingsmead Bookshops (see Amazon).
Arthur Ponsonby, the author of English Diaries (available at Internet Archive), says the journal is often silly and very dull. Jill Rubenstein, author of Lady Mary’s biography in the Oxford Dictionary of National Biography (login needed), says: ‘The journal ranges from banal descriptions of card games and weather to perceptive social observation and expressions of sincere affection, often closely and unselfconsciously juxtaposed. The personality which emerges from the whole combines elements of the mundane and the preposterous with the deeply sympathetic.’
The only extracts of Lady Mary’s diary that I can find online are thanks to Yale University Library (and concern Horace Walpole).
7 September 1767 ‘Mr Walpole called on me at five o’clock in very low spirits; he had received an account of his brother being dangerously ill, and Mr Conway had wrote him word of the dreadful accident the poor Duke of Grafton had had, of one of the horses of his chaise, as he was driving, treading upon a man, of which hurt he was since dead. He left me at six o’clock to go to some engagement.’
29 July 1770
‘We had two beaux besides Mr Walpole at Strawberry Hill, Lord Bristol and Mr Hervey. Lady Greenwich and Lady Sackville came from Sudbrook, Lady Jane Scott from town, and Lady Browne from Twickenham. ’Twas a terrible hot day. We had a great dinner very ill-dressed, yet Mr Walpole had sent for a cook on purpose, who certainly knew but little of his trade: he himself is, as you know, always agreeable and always makes his house so to his company. All went well till the evening, when Lady Blandford arrived, and very soon after the servants told Mr Walpole the Princess Amelia was coming: this put all in confusion. Lady Blandford desired to be shut up, but none of the company agreeing to be shut up with her, she was obliged to remain in the Gallery. I went down to meet the Princess: Lady Powis and Lady Harriot Varnon were with her. H.R.H. went over all the house: the Gallery last, as she did not care to disturb the company, and therefore did not stop to look at any of the pictures, desiring they would set down and not mind her. Those she passed by she spoke to, which were Lady Charlotte Edwine, Lady Greenwich, and Lord Bristol, but unluckily, not seeing Lady Jane Scott, she was not taken notice of, which displeased her very much, and though the Princess sent a message to her by me when she went away, all did not do: she would be offended. Lady Blandford was out of humour at being deprived of Mr Walpole’s company, so the party did not end so well as it promised. Things being in this situation Lady Greenwich called for her chaise, and as I was to lie at Sudbrook I followed her example.’
20 December 1768
‘The new opera, I am told, is extremely disliked. Mr Walpole says he will go to it no more. He made the Princess Amelia a present of his snuff-box with the picture of Harry the Fourth of France, who she was expressing her admiration of. As he had wore it in his pocket for above a year, I don’t think it was proper, at least I should have thrown out the snuff; however, it was very politely received and accepted.’
6 January 1769
‘Before he [Mr Walpole] came in, the Princess showed me the lines he had sent her engraved in the lid of the box; to which she had ordered to be added, that it was given her by the Honourable Horatio Walpole, son of that great Minister, Sir Robert Walpole, Earl of Orford. Nothing, I think, could be more polite to Mr Walpole, and he seemed to be of that opinion when she showed it him, only saying that he was quite ashamed of her goodness.’
25 April 1769
‘Mr Walpole called at my house, and approves of all I have done since he was here. He has given me a design for some frames to be placed over the doors in my book-room, and repeated to me the epilogue he made for Mrs Clive which she spoke last night on quitting the stage. ‘Tis like everything he has ever wrote, extremely pretty. Nobody has his genius. He gave me a play [The Mysterious Mother] of his own writing. I once heard him repeat some scenes that I thought very fine.’
27 April 1769
‘Lady Spencer has lost her little child. Mr Walpole laughed at me for saying I was concerned.’
14 May 1769 ‘I must not forget to mention that on Saturday evening Mr Walpole, who was one of the party, was both uncivil and ill-natured to me, and with no other provocation than saying what almost every other person agrees in that the French Ambassadress was very illbred. Mr Conway with his usual goodness took my part very warmly and seemed hurt at what Mr Walpole had said. As it was with an ill-natured intention, I own it surprised me, and I’m afraid I shall not soon forget it.’
10 January 1784
‘Sir Edward Walpole is dying; he has been declining some months but is now past all hopes of recovery. Mr Walpole will lose a place which Sir Edward held for him and though he has another which is very considerable, ’tis unpleasant to lose anything that one has had for any time. I called on him this morning but he could not see me.’
30 January 1785
‘I passed an hour with Mr Walpole this evening and was surprised to find him so much recovered though still weak; he told me he had a bad fall the day before by imprudently rising from his chair without his stick and hurt himself so much that he imagined it would bring the gout again but contrary to his expectations he had slept the whole night and was quite well in the morning.’