The International Commission on Mathematical Instruction, ICMI, was first established at the International Congress of Mathematicians held in Rome in 1908, on the suggestion of the American mathematician and historian of mathematics David Eugene Smith. The first President of ICMI was Felix Klein and the first Secretary-General was Henri Fehr. From the very beginning, the international journal L'Enseignement Mathématique, founded in 1899 by Henri Fehr and Charles Laisant, was adopted as the official organ of ICMI - which it is still today. ICMI also publishes, under the editorship of the Secretary, a Bulletin appearing twice a year. (Starting with Bulletin No. 39, December 1995, the ICMI Bulletin is accessible on the internet.)
After an interruption of activity between the two World Wars, ICMI was reconstituted in 1952, at a time when the international mathematical community was being reorganized, as an official commission of the International Mathematical Union, IMU. This defines the formal position of ICMI also today. Thus, the Terms of Reference of ICMI are established by the General Assembly of IMU, which is also responsible for the election of the Executive Committee of the Commission. Furthermore, the far majority of the funding of ICMI comes from IMU.
As a scientific union, IMU is a member organization of the International Council of Scientific Union, ICSU. This implies that ICMI, through IMU, is to abide to the ICSU statutes, one of which establishes the principle of non-discrimination. This principle affirms the right and freedom of scientists to associate in international scientific activities regardless of citizenship, religion, political stance, ethnic origin, sex, and suchlike. Apart from observing general IMU and ICSU rules and principles, ICMI works with a large degree of autonomy.
More information about the history of ICMI can be found in a paper by former ICMI Secretary A. Geoffrey Howson:
A.G. Howson, "Seventy-five years of the International Commission on Mathematical Instruction", Educational Studies in Mathematics 15 (1984) 75-93
or in the book by former IMU Secretary Olli Lehto:
O. Lehto, Mathematics Without Borders: A History of the International Mathematical Union, Springer-verlag, 1998. ISBN 3-540-98358-9