Mary Botham was educated at home, and read widely; she commenced writing verses at a very early age. Together with her husband she wrote over 180 books.

On 16 April 1821 she was married in Uttoxeter to William Howitt, and began a career of joint authorship with her husband. They lived initially in Heanor in Derbyshire where William was a pharmacist. It was not until 1823, when they were living in Nottingham, that William decided to give up his business with his brother Richard and concentrate with Mary on writing. Their literary productions at first consisted chiefly of poetical and other contributions to annuals and periodicals, of which a selection was published in 1827 under the title of The Desolation of Eyam and other Poems. William and Mary mixed with many of the important literary figures of the day including Charles Dickens, Elizabeth Gaskell and Elizabeth Barrett Browning. In 1837 they went on a tour of the north and stayed with William and Dorothy Wordsworth. Their work was well regarded, as can be seen from the minister George Byng's present in 1839 from Queen Victoria. She gave him a copy of Mary's book Hymns and Fireside Verses. In the same year, her brother-in-law Godfrey Howitt set out with his wife and her family to emigrate to Australia, arriving at Port Philip in April 1840. The life of Mary Howitt was completely bound up with that of her husband; she was separated only from him during the period of his Australian journey (1851-4). On removing to Esher in 1837 she commenced writing her well-known tales for children, a long series of books which met with signal success. They moved to London in 1843, and following a second move in 1844 they counted Tennyson amongst their neighbors.

While residing at Heidelberg in 1840 her attention was directed to Scandinavian literature, and in company with her friend Madame Schoultz she set herself to learn Swedish and Danish. She afterward translated and introduced Fredrika Bremer's novels (1842-1863, 18 vols.) to English readers. Moreover, Howitt also translated many of Hans Christian Andersen's tales, such as

Among her original works were The Heir of West Way Ian (1847). She for three years she edited the Drawing-room Scrap Book, writing (among other articles that would be included therein) "Biographical Sketches of the Queens of England". She edited the Pictorial Calendar of the Seasons, translated Ennemoser's History of Magic, and took the chief share in The Literature and Romance of Northern Europe (1852). She also produced a Popular History of the United States (2 vols. 1859), and a three-volume novel called The Cost of Caergwyn (1864).

In June 1852, the three male members of the family, accompanied by Edward La Trobe Bateman, sailed for Australia in the hope of finding a fortune. William would be reunited with his brother Godfrey Howitt, while Mary and her two daughters, the elder, Margaret, who had just returned from a year in Munich with Kaulbach (this adventure was later published as a book) moved into the Hermitage, Mr Bateman's cottage in Highgate. This had previously been occupied by Dante Gabriel Rossetti.

In 1851, her husband and her two sons traveled to Australia in the hope of finding their fortune, but they returned a number of years later; William wrote a number of books describing the flora and fauna of Australia. Her son, Alfred William Howitt, was to be renowned as an Australian explorer, anthropologist and naturalist and the discoverer of the remains of the explorers Burke and Wills, which he brought to Melbourne for burial.

Other children included: Herbert Charlton Howitt, who was drowned while engineering a road in New Zealand; Anna Mary Howitt, wife of Alfred Alaric Watts, the biographer of her father, and author of Art Work in Munich [sic, this is an error in the dictionary cited, correct is An Art-Student in Munich], who died at Dietenheim on 23 July 1884; and Margaret Howitt, the writer of the Life of Fredrika Bremer and of the memoir of her own mother.

Her name was attached as author, translator, or editor to upwards of 110 works. She received a silver medal from the Literary Academy of Stockholm, and on 21 April 1879 was awarded a civil list pension of £100. a year. In the decline of her life she joined the church of Rome, and was one of the English deputation who were received by Pope Leo XIII on 10 January 1888. Her interesting Reminiscences of my Later Life were printed in Good Words in 1886. The death of her husband in 1879, and of her eldest child, Mrs. A. A. Watts, in 1884, caused her intense grief. The Times says, speaking of the Howitts:

Their friends used jokingly to call them William and Mary, and to maintain that they had been crowned together like their royal prototypes. Nothing that either of them wrote will live, but they were so industrious, so disinterested, so amiable, so devoted to the work of spreading good and innocent literature, that their names ought not to disappear unmourned.

Mary Howitt was away from her residence in Meran in Tyrol spending the winter in Rome when she died of bronchitis on 30 January 1888.