Around the time she met Mill, she was, or began, also writing a series of unpublished pieces on women's rights, ethics, toleration and marriage.

Around eighteen months later, something made Taylor break off their friendship,[9] causing Mill to write her a passionate love letter in French (the only piece of correspondence surviving from either for this period of their relationship) in which he refuses to accept her 'eternal adieu', and, although saying 'her wish is his command', insists 'her path and mine are separated, she says; but they can, they must, meet again'.

[14] She ends the essay by saying 'It is for you' (i.e. Mill) 'the most worthy apostle of all the loftiest virtues – to teach such as may be taught, that the higher the kind of enjoyment, the greater the degree',[15] and it is noteworthy that Mill is known, in his much-later essay Utilitarianism, for introducing the concept of differences in the quality of pleasures to a previously quantitative 'hedonic calculus' inherited from Jeremy Bentham.

[16] Despite evidently being extremely happy there with Mill, Taylor was conscience-stricken regarding her husband, keenly feeling the pain, and possible public humiliation, she was putting him through.

[20][21][22] Taylor Mill began publishing her works when she was pregnant with her third child, Helen (who also went on to be a campaigner for women's rights, particularly regarding education and the vote).

She was the co-author of several newspaper articles on domestic violence published anonymously in the Morning Chronicle, Daily News, and Sunday Times in the 1840s.

[23] She read and commented on all the material produced by John Stuart Mill, who called her a valuable contributor to much of his work, especially On Liberty, which he later dedicated to her when it was published in 1859, a year after her death.

Thomas Carlyle described her as "full of unwise intellect, asking and re-asking stupid questions", while Jane considered her "A peculiarly affected and empty body".

Certainly, he was very reluctant that her name be linked with Mill's in public during his lifetime, even in a suggested dedication to Principles of Political Economy in 1848.

[29] Harriet Taylor Mill died in the Hotel d'Europe in Avignon, France, on 3 November 1858, after developing severe lung congestion.

Upon her death, John Stuart Mill wrote: Were I but capable of interpreting to the world one half the great thoughts and noble feelings which are buried in her grave, I should be the medium of a greater benefit to it, than is ever likely to arise from anything that I can write, unprompted and unassisted by her all but unrivalled wisdom.