Marjorie Ethel Reeves was born in 1905 in the village of Bratton in Wiltshire to a prominent local Baptist family. Reeves read History at St. Hugh's College, Oxford, and later completed a PhD from London University on mediaeval mysticism and heresy. She returned to Oxford in 1938 where she took up a post as Tutor in History for the Society of Oxford Home Students, which later became St. Anne's College. Her work on Abbot Joachim of Fiore and on prophecy and millenarianism in the Middle Ages and in later centuries brought her an international reputation. Reeves had a long association with St Anne's College, as a fellow and administrator, serving as vice-principal of the college from 1951 to 1962 and from 1964-1967. She was made an honorary fellow of the College on her retirement in 1972. Reeves was elected a fellow of the British Academy in 1974 and in 1993 was awarded the Medlicott Medal for her contribution to improving history education. In 1996 she was made a Commander of the Order of the British Empire (CBE).

After her retirement, Reeves continued to research and to write. She discovered a large collection of family correspondence and manuscripts in the house of her aunt, Jane Saffery Whitaker, relating to the history of the Whitaker, Saffery, Attwater, Brinkworth and Reeves families, as well as some material concerning the Steele family. Reeves used this collection as the basis for her book on family history, Sheep Bell & Ploughshare, Moonraker Press, 1978 and in Pursuing the Muses, Leicester University Press, 1997 and other publications.