In 1940, the Strategic Scientific Reserve (henceforth called the SSR) was created as a counter to the Nazi's special Weapons division, HYDRA. While the SSR was an American agency, it contained some of the finest minds from all Allied nations. As we all know, their efforts were largely successful and helped contribute to winning the war against the AXIS powers.
With the formation of of NATO in 1949, the member states saw a need for an agency with both investigative and enforcement powers that could operate within the territory of the organization without needing to worry about jurisdictional red tape. The Supreme Headquarters, International Espionage, Law-Enforcement Division (henceforth referred to as SHIELD) was created the following year. As with all such agencies, there were controversies, conflicts, SNAFUs, and even some corruption which led to a succession of directors. As control and funding of the agency became a political battle among the member states, SHIELD became less and less effective until the United Nations stepped in. Officially, they created their own agency and hired many, even most, of the agents and staff of SHIELD.
The Strategic Hazard Intervention Espionage Logistics Directorate was born, coincidentally called SHIELD. It is fully funded and overseen by the United Nations and the Director reports directly to the Secretary General though so too does the committee which keeps an eye on SHIELD's activities.
The current Director is one Nicholas Joseph Fury.