Mary Fairfax, as she was born in Jedburgh, Scotland, was the daughter of a naval officer. She received precisely one year of formal education before her marriage in 1804 to a cousin who was a captain in the Russian navy. After his death in 1807 she married another cousin, W. Somerville, an army physician, in 1812.

Somerville was unique in 19th-century British science because she was an independent female. Virtually all other women participated in science as the wife or sister of a husband or brother whom they assisted and sometimes went on to make some small contribution of their own. Her interest in science began when as a young girl she first heard of algebra and Euclid and satisfied her curiosity as to the nature of these subjects from books she purchased. She certainly received no encouragement from her father nor was her first husband much more sympathetic. She was more fortunate with her second husband, who encouraged and assisted her.

Living with her husband in London from 1816 she soon became a familiar and respected figure in the scientific circles of the capital. Her first significant achievement was her treatise on the Mécanique céleste (Celestial Mechanics) of Pierre Simon de Laplace. She was persuaded to undertake this difficult task by John Herschel and in 1831 750 copies of The Mechanism of the Heavens were published. The work was a great success and was used as a basic text in advanced astronomy for the rest of the century.

She followed this in 1834 with her On the Connexion of the Physical Sciences, a more popular but still serious work. In this she suggested that the perturbations of Uranus might reveal the existence of an undiscovered planet. Somerville was of course denied such obvious honors as a fellowship of the Royal Society as a result of her work. She was, however, granted a government pension of £300 a year in 1837.

From 1840, because of the health of her husband, she moved to Europe living mainly in Italy. It was there that she produced her third and most original work, Physical Geography (1848). It was widely used as a university text book to the end of the century, although overshadowed by the Kosmos (Cosmos) of Friedrich von Humboldt, which came out in 1845.

She produced her fourth book, On Molecular and Microscopic Science (1869), at the age of 89 and was working on a second edition when she died.

When a hall was opened in Oxford in 1879 for the education of women it was appropriately named for her and was to produce sufficient talent to refute her own belief that genius was a gift not granted to the female sex.