She was the fifth and youngest daughter of the soldier and politician John Campbell, 2nd Duke of Argyll (1680–1743), and his second wife, Jane (c.1683–1767), a maid of honour to Queen Anne and Caroline, Princess of Wales.

Already having received a handsome legacy from her father, she set out on her life of independence (she never remarried), that became (as her entry in the Dictionary of National Biography puts it) "marked by gossip, travel, devotion to royalty, and self-imposed misadventure".

In her grandiose shows of grief on the death of Prince Edward, Duke of York and Albany, in 1767,[5] Lady Mary alleged in veiled hints that they had been secretly married, a claim that brought her further derision.

[citation needed] Mary, however, did not see that this predicament had been self-inflicted and from then on saw any disaster – servants' incompetence, unsuccessful auction bids, rheumatism – as part of a Maria Theresa-instigated plot pursuing her across Europe.

Though devoted and mock-gallant in his flattery of her (his The Castle of Otranto in 1765 was dedicated to her), Walpole also believed that she had a lack of a sense of humour and pride in her own self-importance which made most of her misfortunes self-inflicted.

He called her and two of her sisters (Caroline Townshend, Baroness Greenwich, and Lady Betty Mackenzie) the three furies, and wrote elsewhere: 'She was much a friend of mine, but a later marriage,[7] which she particularly disapproved, having flattered herself with the hopes of one just a step higher, has a little cooled our friendship.

However ... Lady Mary has a thousand virtues and good qualities: she is noble, generous, high-spirited, undauntable, is most friendly, sincere, affectionate, and above any mean action.

She was a frequent visitor to the Houses of Commons and Lords, witnessing political controversies such as Warren Hastings's trial and the debate over the Cumberland election petition in 1768 (in which she backed Sir James Lowther).

Her Oxford Dictionary of National Biography (DNB) entry states: "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.