Born on November 27, 1873, in Coleford, Gloucester, England, Eaton received formal training in art at the Taunton School of Art in Somerset, England, which awarded her an "Art Class Tea Certificate" and an "Art Master's Certificate." She also studied art at the Royal College of Art in South Kensington, and at Chelsea Polytechnic, both in England.She worked as a painter of Worcester porcelain for a while, before traveling to Jamaica in 1909, where she stayed for two years, and painted butterflies and moths. In June of 1911 she went to New York City to visit friends but ended up staying until 1932, working as an artist for the New York Botanical Garden. She was the principal illustrator for the Garden's illustrated serial Addisonia (preparing over 80% of the 800 plates) and she did the vast majority of paintings and line drawings used to illustrate Britton and Rose's The Cactaceae (1919-1923). Other scientific illustrations during this time appearde in the National Geographic Magazine and in the Contributions from the U.S. National Herbarium.
In 1922, she received the silver-gilt Grenfelt Medal of the Royal Horticultural Society, London.
Her work is in the permanent collections of the National Geographic Society (over 600 botanical watercolors), The New York Botanical Garden, and the Smithsonian Institution (most of the originals of Britton & Rose's The Cactaceae). The Hunt Institute for Botanical Documentation also has a few examples of her work, as well as plates on indefinite loan from the Smithsonian.
Mary Emily Eaton died in Cossington, Somerset, England on 4 August 1961.