The sixth of eight children, Harriet Martineau was born in Norwich, England, where her father was a manufacturer. The family was of Huguenot extraction and professed Unitarian views. Her brother, James Martineau, was a clergyman of some note in the tradition of the English Dissenters. The atmosphere of her home was industrious, intellectual and austere; she herself was clever, but weakly and unhappy; she had no sense of taste or smell, and moreover grew deaf while young, having to use an ear trumpet. At the age of sixteen the state of her health and nerves led to a prolonged visit to her father's sister, Mrs Kentish, who kept a school at Bristol. Here, in the companionship of amiable and talented people, her life became happier. Here, also, she fell under the influence of the Unitarian minister, Dr Lant Carpenter, from whose instructions, she says, she derived "an abominable spiritual rigidity and a truly respectable force of conscience strangely mingled together." From 1819 to 1830 she again resided chiefly at Norwich. About her twentieth year her deafness became confirmed. In 1821 she began to write anonymously for the Monthly Repository, a Unitarian periodical, and in 1823 she published Devotional Exercises and Addresses, Prayers and Hymns.

In 1826 her father died, leaving a bare maintenance to his wife and daughters. His death had been preceded by that of his eldest son, and was shortly followed by that of a man to whom Harriet was engaged. Mrs Martineau and her daughters soon after lost all their means by the failure of the house where their money was placed. Harriet had to earn her living, and, being precluded by deafness from teaching, took up authorship in earnest. Besides reviewing for the Repository she wrote stories (afterwards collected as Traditions of Palestine), gained in one year (1830) three essay prizes of the Unitarian Association, and eked out her income by needlework. In 1831 she was seeking a publisher for a series of tales designed as Illustrations of Political Economy. After many failures she accepted disadvantageous terms from Charles Fox, to whom she was introduced by his brother, the editor of the Repository. The sale of the first of the series was immediate and enormous, the demand increased with each new number, and from that time her literary success was secured.

In 1832 she moved to London, where she numbered among her acquaintances Henry Hallam, Harriet Taylor, Alexander Maconochie, Henry Hart Milman, Thomas Malthus, Monckton Milnes, Sydney Smith, John Stuart Mill, George Eliot, Edward George Bulwer-Lytton, Elizabeth Barrett Browning and later Thomas Carlyle. Florence Nightingale and Charlotte Brontë also became her friends. Until 1834 she continued to be occupied with her political economy series and with a supplemental series of Illustrations of Taxation. Four stories supporting the Whig Poor Law reforms came out about the same time. These tales (direct, lucid, written without any appearance of effort, and yet practically effective) display the characteristics of their author's style. Tory paternalists reacted by calling her a Malthusian "who deprecates charity and provision for the poor", while Radicals opposed her to the same degree. Whig high society fêted here. In May 1834 Charles Darwin got a letter from his sisters telling him that Martineau was "a great Lion in London" and recommending Poor Laws and Paupers Illustrated in pamphlet sized parts. They added that their brother "Erasmus knows her & is a very great admirer & every body reads her little books & if you have a dull hour you can, and then throw them overboard, that they may not take up your precious room."

In 1834, when she completed the series, Harriet Martineau paid a long visit to the United States. Here her open adhesion to the Abolitionist party, then small and very unpopular, gave great offence, which was deepened by the publication, soon after her return, of Theory and Practice of Society in America (1837) and a Retrospect of Western Travel (1838). An article in the Westminster Review, "The Martyr Age of the United States", introduced English readers to the struggles of the Abolitionists.

After the Voyage of the Beagle Charles went in October 1836 to stay with his brother Erasmus Alvey Darwin in London, and found Erasmus spending his days "driving out Miss Martineau". The Darwins shared her Unitarian background and Whig politics, but their father Robert was concerned that as a potential daughter-in-law, her politics were too extreme. He was upset by a piece he read in the Westminster Review calling for the radicals to break with the Whigs and give working men the vote "before he knew it was not hers, and wasted a good deal of indignation, and even now can hardly believe it is not hers." Charles Darwin called on Martineau and remarked that "she was very agreeable, and managed to talk on a most wonderful number of subjects, considering the limited time", which included the social and natural worlds she was then writing about in her book Society in America, including the "grandeur and beauty" of the "process of world making" she had seen at Niagara Falls. He added that "I was astonished to find how ugly she is" and "she is overwhelmed with her own projects, her own thoughts and abilities", though "Erasmus palliated all this, by maintaining one ought not to look at her as a woman." For her part, Martineau described Darwin as "simple, childlike, painstaking, effective". After a later meeting when he was struggling with his own writing and she was starting Deerbrook he expressed astonishment at the ease with which she wrote such fluent prose, and "never has occasion to correct a single word she writes", though she was "not a complete Amazonian, & knows the feeling of exhaustion from thinking too much."

She followed the American books with a three volume novel, Deerbrook (1839) – a story of middle class country life with a surgeon hero. To the same period belong a few little handbooks, forming parts of a Guide to Service. The veracity of her Maid of All Work led to a widespread belief, which she regarded with some complacency, that she had once been a maid of all work herself.

In 1839, during a visit to Continental Europe, Harriet Martineau's health broke down. Her chronic ill-health was due to an ovarian cyst, and she visited her brother-in-law, the celebrated Newcastle upon Tyne doctor Thomas Michael Greenhow on several occasions to try to alleviate her symptoms - on the last occasion staying for six months in the family house at 28 Eldon Square. She then moved down-river to Tynemouth, where she stayed at Mrs Halliday's boarding -house, 57 Front Street for nearly five years from 16 March 1840. A plaque marks the house where she produced at least three books, including a novel about the Haitian slave leader Toussaint L'Ouverture, and Life in the Sick-Room, describing her life in Tynemouth. She also devotes some hundred pages of her autobiography to this period. Notable visitors included Richard Cobden and Thomas Carlyle and his wife. Harriet had expected to remain an invalid for the rest of her life and delighted in the freedom her telescope allowed. Across the Tyne was the sandy beach ″where there are frequent wrecks - too interesting to an invalid... and above the rocks, a spreading heath, where I watch troops of boys flying their kites; lovers and friends taking their breezy walks on Sundays..." She also gives a lyrical picture of Tynemouth.

The busybody Mrs Jellyby in Bleak House is probably based on Harriet Martineau, who nevertheless retained her high regard for Charles Dickens.

During her illness she for a second time declined a pension on the civil list, fearing to compromise her political independence. Her letter on the subject was published, and some of her friends raised a small annuity for her soon after. In 1844 Harriet Martineau underwent a course of mesmerism, returning to health after a few months. She eventually published an account of her case, which had caused much discussion, in sixteen Letters on Mesmerism. This led to friction with 'the natural prejudices of a surgeon and a surgeon's wife' and in 1845 she left Tynemouth for Ambleside in the Lake District, where she built herself "The Knoll", the house in which the greater part of her later life was spent. In 1845 she published three volumes of Forest and Game Law Tales. In 1846 she made a tour with some friends in Egypt, Palestine and Syria, and on her return published Eastern Life, Present and Past (1848). This travelogue showed that as humanity passed through one after another of the world's historic religions, the conception of the Deity and of Divine government became at each step more and more abstract and indefinite. The ultimate goal Harriet Martineau believed to be philosophic atheism, but this belief she did not expressly declare. It described ancient tombs, "the black pall of oblivion" set against the paschal "puppet show" in the Church of the Holy Sepulchre, with the message that Christian beliefs in reward and punishment were based on heathen superstitions. Describing an ancient Egyptian tomb, she wrote "How like ours were his life and death!.. Compare him with a retired naval officer made country gentleman in our day, and in how much less do they differ than agree!" The book's "infidel tendency" was too much for the publisher John Murray, who rejected it.

She published at about this time Household Education, expounding the theory that freedom and rationality, rather than command and obedience, are the most effectual instruments of education. Her interest in schemes of instruction led her to start a series of lectures, addressed at first to the school children of Ambleside, but afterwards extended, at their own desire, to their elders. The subjects were sanitary principles and practice, the histories of England and North America, and the scenes of her Eastern travels. At the request of Charles Knight she wrote, in 1849, The History of the Thirty Years' Peace, 1816–1846 – an excellent popular history written from the point of view of a "philosophical Radical", completed in twelve months.

Harriet Martineau edited a volume of Letters on the Laws of Man's Nature and Development, published in March 1851. Its form is that of a correspondence between herself and the garrulous self-styled scientist Henry G. Atkinson, and it expounds that doctrine of philosophical atheism to which Miss Martineau in Eastern Life had depicted the course of human belief as tending. The existence of a first cause is not denied, but is declared unknowable, and the authors, while regarded by others as denying it, certainly considered themselves to be affirming the doctrine of man's moral obligation. Atkinson was a zealous exponent of mesmerism. The prominence given to the topics of mesmerism and clairvoyance heightened the general disapprobation of the book, which outraged literary London with its mesmeric evolutionary atheism, causing a lasting division between Harriet Martineau and some of her friends.

She contributed regularly to the Daily News from 1852 to 1866, writing sometimes six leaders a week. Her Letters from Ireland, written during a visit to that country in the summer of 1852, appeared in that paper. She was for many years a contributor to the Westminster Review, and was one of the little band of supporters whose pecuniary assistance in 1854 prevented its extinction or forced sale.

In the early part of 1855 Harriet Martineau found herself suffering from heart disease. She now began to write her autobiography, but her life, which she supposed to be so near its close, was prolonged for twenty years. Her two-volume autobiography was published posthumously in 1877. She cultivated a tiny farm at Ambleside with success, and her poorer neighbours owed much to her. Her busy life bore the consistent impress of two leading characteristics – industry and sincerity.

When Charles Darwin's book The Origin of Species was published in 1859, Erasmus Alvey Darwin sent a copy to his old flame Harriet Martineau, who at 58 was still reviewing from her home in the Lake District. From her "snow landscape" Martineau sent her thanks, adding that she had previously praised "the quality & conduct of your brother's mind, but it is an unspeakable satisfaction to see here the full manifestation of its earnestness & simplicity, its sagacity, its industry, & the patient power by which it has collected such a mass of facts, to transmute them by such sagacious treatment into such portentous knowledge. I should much like to know how large a proportion of our scientific men believe he has found a sound road." She wrote to her fellow Malthusian (and atheist) George Holyoake enthusing "What a book it is! – overthrowing (if true) revealed Religion on the one hand, & Natural (as far as Final Causes & Design are concerned) on the other. The range & mass of knowledge take away one's breath." To Fanny Wedgwood she wrote "I rather regret that C.D. went out of his way two or three times to speak of "The Creator" in the popular sense of the First Cause.... His subject is the "Origin of Species" & not the origin of Organisation; & it seems a needless mischief to have opened the latter speculation at all – There now! I have delivered my mind."

As early as 1831, Martineau wrote on the subject "Political Economy" (as the field of economics was then known). Her goal was to popularize and illustrate the principles of laissez faire capitalism, though made no claim to original theorising.

Martineau's reflections on Society in America published in 1837 are prime examples of her approach to the area later known as sociological methods. Her ideas in this field were set out in her 1838 book How to Observe Morals and Manners. She believed that some very general social laws influence the life of any society, including the principle of progress, the emergence of science as the most advanced product of human intellectual endeavor, and the significance of population dynamics and the natural physical environment.

Auguste Comte coined the name sociology and published a rambling exposition under the title of 'Cours de Philosophie Positive' in 1839. Martineau undertook a translation that was published in two volumes in 1853 as The Positive Philosophy of Auguste Comte (freely translated and condensed by Harriet Martineau). It was a remarkable and difficult achievement, but a successful one. Comte himself recommended these volumes to his students instead of his own. Some writers regard Martineau herself as "the first woman sociologist". Her introduction of Comte to the English-speaking world and the elements of sociological perspective that may be found in her original writings argue for her recognition as a kindred spirit, if not a significant contributor.

Harriet Martineau died at "The Knoll" on 27 June 1876. The verdict which she recorded on herself in the autobiographical sketch left to be published by the Daily News has been endorsed by posterity. She wrote "Her original power was nothing more than was due to earnestness and intellectual clearness within a certain range. With small imaginative and suggestive powers, and therefore nothing approaching to genius, she could see clearly what she did see, and give a clear expression to what she had to say. In short, she could popularize while she could neither discover nor invent."