Day was born on 22 June 1748 London, the only child of Thomas and Jane Day. Thomas Day, Sr. died when Day was about a year old, leaving him fatherless but wealthy. Day first attended a school in Stoke Newington, Middlesex, but after a bout with smallpox he removed to Charterhouse School. He subsequently attended Corpus Christi College, Oxford where he became a star debater and developed a close friendship with William Jones, but he did not graduate and left the college in 1767.

Day moved back to his family estate at Barehill, Berkshire. There he met the progressive educator Richard Lovell Edgeworth, from whom he became almost inseparable. Together the resolved to educate Edgeworth's son, Dick, in the style of Jean-Jacques Rousseau's Emile. Edgeworth and the project converted Day to Rousseauism. He declared in 1769 that the two books he would save, were all the world's books to be destroyed, would be the Bible and Emile. He, Edgeworth and Dick even visited Rousseau in France. Because of his connection with Edgeworth, Day was able to join the Lunar Society in Lichfield and meet and converse with Erasmus Darwin as well as Anna Seward.

After this education project, Day undertook a second: he tried to train a wife. After failing to find the perfect wife (several women turned down his proposals of marriages), he decided to adopt two foundlings from orphanages and, using Rousseau's maxims, educate them to be the perfect wife (two would ensure that one of them worked out). He adopted a 12-year-old and an 11-year-old whom he renamed Sabrina and Lucretia and took them to France to educate them in isolation. Unfortunately, the girls became ill and "squabbled" and he decided to give up on Lucretia, who he did not think could satisfy him intellectually. Sabrina he felt was still a possibility, but her character had to be further strengthened. After dropping hot wax on her arms and hearing her scream, though, he gave up in despair.

Although Day was wealthy, he decided to study the law and in 1776 was admitted to Lincoln's Inn; he rarely practised.

Day did finally meet his "paragon" of a woman in Esther Milnes (1753-1792), an heiress from Chesterfield. They were married on 7 August 1778. The couple subsequently moved to a small estate at Stapleford Abbotts, near Abridge in Essex. They lived a very ascetic lifestyle and Esther was never allowed to contact her family. In 1780, the couple moved to Anningsley in Surrey, when Day bought a new estate there. It was a philanthropic project for both husband and wife and they labored to improve the conditions of the working classes around them.

In 1773, Day published his first work-"The Dying Negro," a poem he had written with John Bicknell that tells the horrifying story of a runaway slave. It was a bestseller and one of the first pieces of literature in the campaign to abolish the slave trade in Britain.

The contradiction between the claim that "all men are created equal" and the existence of American slavery, attracted comment when the United States Declaration of Independence was first published. Congress having made a few changes in wording, deleted nearly a fourth of the draft before publication, most notably removing a passage critical of the slave trade, and there were members of Congress who owned black slaves. In 1776, Thomas Day responding to the hypocrisy in the Declaration wrote;

"If there be an object truly ridiculous in nature, it is an American patriot, signing resolutions of independency with the one hand, and with the other brandishing a whip over his affrighted slaves."

Day argued for the rights of the American colonists in his poem "The Devoted Legions" (1776) and in 1780 he argued in Parliament for an early peace with the revolutionaries as well as parliamentary reform. His speeches were also published as pamphlets.

But it was as a writer for children that Day made his reputation. The History of Little Jack (1787) was extremely popular, but it could not match the sales of The History of Sandford and Merton (1783, 1786, 1789) which was a bestseller for over a hundred years. Embracing Rousseau's dictates in many ways, it narrates the story of the rich, noble but spoiled Tommy Merton and his poor but virtuous friend Harry Sandford. Through trials and stories, Harry and the boys' tutor teach Tommy the importance of labor and the evils of the idle rich.

Day was thrown from his horse while trying to break it using kindness at Barehill, Berkshire, on 28 September 1789 and died almost instantly. He was buried at St Mary's Church, Wargrave, Berkshire.

• Blackman, John. A Memoir of the Life and Writings of Thomas Day, author of “Sandford and Merton.” London: John Bedford Leno, 1862. No ISBN available.

• Chandler, Anne. “Defying ‘Development’: Thomas Day’s Queer Curriculum in Sandford and Merton.” Novel Gazing: Queer Readings in Fiction. Ed. Eve Kosofsky Sedgwick. Durham: Duke University Press, 1997.

• Gignilliat, George Warren, Jr. The Author of Sandford and Merton: A Life of Thomas Day, Esq. New York: Columbia University Press, 1932. No ISBN available.

• Rowland, Peter. The Life and Times of Thomas Day, 1748-1789: English Philanthropist and Author, Virtue Almost Personified. Lewiston: E. Mellen Press, 1996. ISBN 0773488448.

• Scott, S. H. The Exemplary Mr Day, 1748-1789, author of Sandford and Merton, A Philosopher in Search of the Life of Virtue and of a Paragon Among Women. London: Faber and Faber Ltd., [1935]. No ISBN available.

• Uglow, Jenny. The Lunar Men: Five Friends Whose Curiosity Changed the World. New York: Farrar, Straus and Giroux, 2002.