David Christie Martin (1914-76) was the Assistant Secretary (1947-62) and Executive Secretary (1962-76) of the Royal Society. During his long tenure he oversaw the modernization and expansion of the Society's administration, finances, publications and premises, and worked closely with the Officers, Council and the Society's many subcommittees. He was closely involved with the national and international aspects of the Society's work, and with the Fellows, visitors and external relations at all levels. The key link between the Royal Society and Whitehall, he developed strong informal contacts with civil servants in the Treasury, other government departments and the research councils, which greatly facilitated the Society's work. He was a significant point of continuity in the administration and governance of the Society over this long period, yet it is remarkable that we know little of Martin's work. Drawing on Martin's diary for 1947- 49, recently unearthed at the Royal Society Library, this paper gives an account of his activities in the Royal Society and in postwar scientific London in this period. In so doing it sheds new light on British science at the beginning of the Cold War, and on the key role of the 'invisible administrator' in modern science. © 2012 The Royal Society.