Eileen Power was the eldest of three daughters of a stockbroker who was imprisoned for fraud in 1891. Their mother died when Eileen was 14, and Eileen took on the responsibility of caring for her sisters, to whom she was very close, especially Rhoda, who grew up to be a children's writer and broadcaster. Their maternal aunts took the girls to live in Oxford so they could attend the best of the Girls Public Day School Trust schools. Eileen won a scholarship to Cambridge University, from which she graduated with a First, and then studied at the Sorbonne and the the École nationale des chartes. A critic of British foreign policy, she was an active member of the Union of Democratic Control during World War I, along with Charles Trevelyan, Ramsay MacDonald, J.A. Hobson, Lady Ottoline Morrell, Israel Zangwill, Margaret Llewelyn Davies, and Olive Schreiner. Her first book, The Paycockes of Coggeshall, was published in 1919. She was named lecturer in Political Science at the London School of Economics, and reader of the University of London. In 1931 she was appointed Professor of Economic History at the London School of Economics, only the second woman to hold such a position, and remained there until 1938, when she became Professor of Economic History at Cambridge University. She became a leading medieval historian of the interwar years, and was influential in the development of the field of economic history. Her most famous work was Medieval People, published in 1924. In 1927, she founded the Economic History Review. In 1933, she joined William Beveridge, head of the London School of Economics, in establishing the Academic Freedom Committee, an organization that helped academics fleeing Nazi Germany. In 1937, she married historian Michael Postan, who was about 10 years her junior. She died of a heart attack at age 51. Her last book, The Wool Trade in English Medieval History (1941), was published posthumously.