The Secretaries were the Society's principal executive officers, responsible directly to the Committee. Initially there were three: one Anglican, one Free Churchman and one resident foreigner, all ordained men. In 1826 the Foreign Secretary resigned, and after several unsuccessful attempts to replace him, the post was discontinued, though an Assistant Foreign Secretary remained until 1895, to deal with foreign-language correspondence. It appears that there was no clear division between the duties of the two remaining Secretaries until about 1890, when the overseas work was divided between them on a geographical basis, countries being grouped into 'A' and 'B' fields. In 1964 a more radical change was made, when one Secretary was assigned to the home work, and to other to the work overseas. In 1966 a third Secretary (a layman) was added, to deal with internal administration, and the Departments in Bible House were grouped into three divisions. In 1973 a 'Senior Mangement Group' was set up, composed of the Secretaries and Heads of Department, and after further changes the 'Executive Director' emerged as the Society's chief officer.
Letters to and from the Secretaries, or written on their behalf, form the Society's main correspondence series. From 1804 to about 1930 letters were filed chronologically and indexed by writer (D1/-4). With some minor exceptions (D2) there was no geographical or subject division of the material. To overcome this problem several series of copybooks were created (D1/5-7), of which only a few survive. In the 1930s the change to subject-based filing was made, and the records thereafter are grouped by topic. Most of the foreign correspondence was arranged by country (D8) . Note that there are large gaps in the surviving series, in particular from 1857 to 1900, and that not all the 20th century material has been catalogued.