John Stuart Mill (1806–73) was born in London. He was the son of James Mill, a philosopher, economist, and historian. James Mill educated his son with the advice and assistance of Jeremy Bentham. Because of his father's friendship with Bentham, the younger Mill was exposed to the ideas of utilitarianism early on.

Mill learned Greek at the age of three, Latin a little later, and by the age of twelve he was a competent logician, having read Aristotle's books on logic. By sixteen he was a well-trained economist. At twenty he suffered a nervous breakdown, which persuaded him that more was needed in life than devotion to the public good and the cultivation of a sharp intellect. He took up a study of the poets Coleridge, Wordsworth, and Goethe, thinking he needed to develop his aesthetic sensibilities.

In 1843 he wrote a System of Logic and in 1848 the Principles of Political Economy. But his books on politics and morals remain fresh among all his work. In 1859 he wrote On Liberty and in 1861 Utilitarianism, which remains the classic defense of the view that humans ought to aim at maximizing the welfare or happiness of all sentient beings.

On Liberty was the most controversial work he wrote in his lifetime. The book was inspired by letters he and his wife Harriet Taylor wrote, in which they expressed how adventurous individuals were becoming all too rare and that societies like America and Great Britain cared more for conformity than individual liberty. In On Liberty he writes that there is but one justification for society to interfere in the lives of its citizens — to defend others from harm. This has come to be known as Mill's “harm principle.”