Margaret King was born into the Anglo-Irish Kingsborough family, leading members of the Protestant Ascendancy, the Anglican landed elite in Ireland who cooperated with the British Crown in governing the Kingdom.

The eldest son, Stephen Moore, 3rd Earl Mount Cashell, went on to graduate from Trinity College, Cambridge,[8] marry a Swiss woman, and live in several countries.

When, on the eve of the 1798 rebellion, Fitzgerald received a wound in the course of his arrest (which was to prove fatal), she intervened to prevent the news from reaching his pregnant wife, in the hope that his condition might improve and diminish the shock.

[14] The Bishop of Ossory would have had King and others of her female acquaintance in mind when, in a sermon before Earl Camden, the Lord Lieutenant, he decried the progress of revolutionary principles and atheistic philosophy through the "higher ranks" of society.

[16] After the defeat of the insurrection in 1798, Margaret wrote pamphlets opposing the government's policy of abolishing the Irish Parliament and effecting a legislative union with the Kingdom of Great Britain.

[17] Among her extensive circle at this time she counted Lord Cloncurry Valentine Lawless, Charles Fox, Helen Maria Williams, Matilda Tone, and Robert Emmet (fated to hang for attempting to renew the United Irish insurrection in 1803).

Wilmot wrote extensive letters home, some of which were published in 1920 as An Irish peer on the continent (1801–1803) being a narrative of the tour of Stephen, 2nd earl Mount Cashell, through France, Italy, etc.

In the French capital, they met Napoleon, the radical English parliamentarian Charles James Fox and, "up half a dozen flights of stairs, in a remote part of the town", Thomas Paine.

[20] In June 1802 the Cashells had another son, Richard Francis Stanislaus Moore, and Wilmot records that its godparents were William Parnel, "the Polish Countess Myscelska", and the American minister (presumably Robert Livingston, who was in post 1801–1804).

In Rome they were in the company of the Swiss painter, and founding member of the Royal Academy in London, Angelica Kaufmann; the epicure Lord Bristol, Bishop of Derry (who, in the Volunteer crisis of 1783 is said to have imagined himself King of Ireland);[21] the Cardinal Duke of York, brother to the Young Pretender, Charles Edward Stuart; and the Pope, Pius VII, who in his gardens "very gallantly pull’d a hyacinth and gave it to Lady Mount Cashell".

[20] While in Rome, Margaret was introduced to George William Tighe (1776–1837) of Rosanna, Ashford, County Wicklow, an Anglo-Irish gentleman with an interest in agriculture and, in contrast to her husband, with social and political views similar to her own.

[22] In 1813, as Margaret King Moore, she contributed to Stories of Old Daniel, Or, Tales of Wonder and Delight, Containing Narratives of Foreign Countries and Manners, and Designed as an Introduction to the Study of Voyages, Travels, and History in General.

[18] Free with Tighe to follow her own course, in Germany she studied medicine at University of Jena, attending lectures disguised as a man, because medical education was forbidden to women.

In 1823 she published a very popular practical medical guide, Advice to young mothers on the physical education of children, by a grandmother, which went through numerous editions in several countries including Britain and the United States.

[25] Among other un-orthodoxies, in her Advice she insisted on the superiority of female midwives (the competing worldview was the rise of male obstetricians such as William Smellie), and the benefits of the mother herself breastfeeding (as opposed to "throwing" her child on "the bosom of a stranger", i.e. a wet nurse).

But through its central female character, it is also a mediation upon Margaret's own experience as a woman including the pain of an unhappy socially-dictated marriage, and of recuperation through a second relationship enjoyed in relative seclusion.

[29] Margaret is the "lady, the wonder of her kind, whose form was up born by a lovely mind" whom Shelley celebrates in his poem "The Sensitive Plant",[22] and she helped kindle "a new-found sense of radicalism".

That same year, she began hosting a fortnightly salon in her house in Pisa, the Accademia dei Lunatici (Society of Lunatics)[18][34] Those in attendance included the writers Giacomo Leopardi and Giuseppe Giusti, who would play an important role in the Italian patriotic revival.

She was described, in the 1920 introduction to Wilmot's diaries, as "socially charming and attractive, highly cultivated, upright and refined", but "harsh to her children, a Freethinker in religion, and imbued with what were then the most extravagant political notions".