Eileen Power, the eldest of three daughters of Philip Ernest Le Poer Power (1860–1946), stockbroker, and his wife, Mabel Grindlay Clegg (1866–1903), was born at Atrincham on 9th January 1899. Her father was imprisoned for fraud in 1891, and her mother, faced with scandal and financial ruin, moved with her daughters to Bournemouth.
On the death of her mother in 1903, Eileen, Rhoda and Beryl went to live with their grandfather, Benson Clegg, in Oxford. She attended the Oxford High School for Girls.
In 1907 she went to Girton College on a Clothworkers' scholarship, and took a first in both parts of the historical tripos. During her time at the University of Cambridge she joined the National Union of Women's Suffrage Societies. In 1910 she was awarded the Gilchrist research fellowship, and studied at the University of Paris and the École des Chartes. On her return to Britain in 1911 she was awarded the George Bernard Shaw research studentship at the London School of Economics (LSE) where she studied medieval women.
A critic of Britain's foreign policy, Power was an active member of the Union of Democratic Control during the First World War. Fellow members included Charles Trevelyan, Norman Angell, E. D. Morel, Ramsay MacDonald, J. A. Hobson, Charles Buxton, Ottoline Morrell, Philip Morrell, Frederick Pethick-Lawrence, Arnold Rowntree, Morgan Philips Price, George Cadbury, Helena Swanwick, Fred Jowett, Tom Johnston, Bertrand Russell, Philip Snowden, Ethel Snowden, David Kirkwood, William Anderson, Mary Sheepshanks, Isabella Ford, H. H. Brailsford, Israel Zangwill, Margaret Llewelyn Davies, Konni Zilliacus, Margaret Sackville and Olive Schreiner.
Eileen Power's first book, The Paycockes of Coggeshall, was published in 1919. In 1921 she was appointed lecturer in economic history at the London School of Economics. Over the next few years she published Medieval English Nunneries (1922) and Medieval People (1924). Her biographer, Maxine L. Berg, points out that the book "went into ten editions, was the culmination of the first phase of her approach to social history. Its genesis lay in her feminist and pacifist political commitment, and in the methodology she developed of history as literature. The book was a social history deploying literary devices, but even more significantly it was a social history written to spread a message of internationalism."
Kingsley Martin was a fellow teacher at the London School of Economics. He later recalled: "Eileen Power, with whom, like everyone else, I assume was more or less in love. Eileen, indeed, was one of the most attractive women I have ever known. She was good-looking, and carried her erudition as a medieval scholar with wit and grace. She wrote delightfully, her account of the domestic life of nunneries would never bore anyone, and her Medieval People showed that careful scholarship can be made popular and achieve large sales."
Dora Russell was one of her students: "Eileen Power dealt with history. She became distinguished for her fine scholarship and her utter charm, which captivated many of both sexes. We always found it a pleasure to watch her, tall and placid and very much a personality, as she came in to take her place for dinner at high table. She had very beautiful, candid blue-grey eyes."
Eileen worked closely with her sister, Rhoda Power. According to Maxine L. Berg: "With her sister Rhoda she wrote children's history books, of which the most famous was Boys and Girls of History (1926). She was part of literary London, wrote widely in the press, and was a popular lecturer. During the 1920s she also started the memorable BBC schools history broadcasts which she made with Rhoda. The international aspects of medieval history, medieval trade, comparative economic history, and world history, as well as women's and social history, which Eileen Power made her own, were always made immensely attractive and immediately accessible to broad audiences by her extensive use of literary references and personal portraits."In 1927 Power helped to establish the Economic History Review.
Power published The Goodman of Paris in 1928. Three years later she became Professor of Economic History at the London School of Economics. In 1933 she joined William Beveridge in establishing the Academic Freedom Committee, an organization that helped academics fleeing from Nazi Germany. Later that year she published Studies in English Trade in the 15th Century (1933).
Power was a strong opponent of appeasement and according to Maxine L. Berg her radio broadcasts came to an end in 1936 "when she came into conflict with her producers over the political and pedagogical directions of the programmes". Power married the historian Michael Postan, who was ten years her junior, on 11th December 1937.
Eileen Power died of heart failure on 8th August 1940. Her book, The Wool Trade in English Medieval History (1941) was published posthumously. A collection of her lectures, Medieval Women, was published in 1975.