De Beer was the grandson of Dunedin businessman Bendix Hallenstein, founder of Hallenstein Bros., a major New Zealand retailer, and nephew of avid collector Willi Fels, from whom he developed a love of books. The De Beers moved to London in 1910 when Esmond's father Isidore was made manager of Hallensteins' London office. He was educated at Mill Hill School, and in 1914 entered New College, Oxford, to read History. After serving on India's Northwest Frontier during the First World War, he returned home to gain an MA at the University of London in 1923.
De Beer worked as an editor for Clarendon Press, working on two massive and time-consuming projects, first the diary of John Evelyn (six volumes, published in 1955) and then the correspondence of John Locke. The latter work, published in eight volumes between 1976 and 1989, was left uncompleted at the time of De Beer's death.
Though spending much of their life in England, De Beer and his sisters Mary and Dora always regarded New Zealand as their home, and were major benefactors of Dunedin's museums and libraries, most notably to the University of Otago Library, which was the recipient of Esmond's collection of rare books. The De Beers and cousin Charles Brasch were also driving forces behind the inauguration of the university's three arts fellowships: the Robert Burns Fellowship (writing), the Frances Hodgkins Fellowship (art), and the Mozart Fellowship (music). De Beer was also instrumental in helping the Otago Museum's quest to acquire much of his aunt, Agnes Barden's, art collection. Numerous other gifts were given to both of these institutions and to the Dunedin Public Art Gallery. The Dunedin Public Library was also gifted with over 100 rare books, including many first editions of the works of Sir Walter Scott. De Beer also contributed to the library of the University of Essex and the Bodleian Library, among others.
De Beer was a major figure in several notable institutions, including the (British) Historical Association, of which he was vice-president, and the Institute of Historical Research. He served as president of the Hakluyt Society, and was a trustee of the National Portrait Gallery. He was named a fellow of the British Academy in 1965, received a similar honour from University College, London, two years later, and was awarded honorary doctorates by the Universities of Otago, Oxford and Durham.
De Beer died in a nursing home in Stoke Hammond, Buckinghamshire on 3 October 1990, aged 95. Much of his correspondence, and that of his sisters (who predeceased him during the 1980s), is held by the Hocken Collections.
- Notman, Robyn. "Esmond Samuel de Beer". Dictionary of New Zealand Biography. Ministry for Culture and Heritage. Retrieved 23 April 2017.
- Oxford Dictionary of National Biography
- "Esmond Samuel de Beer Dunedin Public Library website.
- McEldowney, W.J. (2002) "Esmond de Beer", University of Otago Library.
- "Portrait of a Gentleman Scholar: Celebrating the Life & Legacy of Esmond de Beer". University of Otago Library. University of Otago. Retrieved 26 May 2015.
- "de Beer family. Community archive.