When the original Old-Age and Survivors Insurance (OASI) Trust Fund was established by the 1939 Social Security Amendments, a Board of Trustees was established for the fund composed of:

• The Secretary of Treasury, designated as Managing Trustee and Chair
• The Secretary of Labor, and
• The Chairman of the Social Security Advisory Board, which after several reorganizations was superceded on the Board of Trustees by the
• Secretary of Health and Human Services.

In 1956 when the Social Security Disability Insurance program (DI) and its associated Trust Fund were established, the existing Board of Trustees was given responsibility for that Trust Fund as well. Thus the Trust Fund became designated as the OASDI Trust Fund.

When Medicare was enacted in 1965, however, a separate Trust Fund with a separate Board of Trustees was created both for the Hospital Insurance and for the Supplemental Medical Insurance programs under Medicare. Even so, the members designated for the two new Boards were the same cabinet officials already serving on the OASDI Board.

Consequently, there are three Boards of Trustees overseeing the Social Security and Medicare programs:

• The Board for the OASDI program;
• The Board for the Hospital Insurance program;
• The Board for the Supplemental Medical Insurance program.

Two Public Trustee positions on all three Boards were created in the 1983 Social Security Amendments, based on a recommendation of the Greenspan Commission aimed at increasing public "confidence in the integrity of the trust funds." The Commissioner of Social Security became a member of all three Boards in the 1994 legislation that made the Social Security Administration an independent agency in the Executive Branch

Today the same six Trustees serve on the three separate Boards of the Old-Age and Survivors Insurance Trust Fund and the Disability Insurance Trust Fund, the Hospital Insurance Trust Fund (Medicare Part A), and the Supplemental Medical Insurance Trust Fund (Medicare Part B).

The two public trustees are nominated by the President, confirmed by the Senate, and may not be from the same political party. They serve for a term of 4 years, but may continue to serve after their term expires until the next annual report is issued or successors are confirmed, whichever occurs first.

Responsibilities of Board of Trustees (in Secs. 201, 1817 and 1840 of the Social Security Act)

• Hold the trust funds
• Report to the Congress each year on past and future status
• Report to the Congress immediately if the amount in either trust fund is too small
• Review policies followed in managing trust funds and recommend changes.

Key policies set by law are that:

• All revenue to trusts funds is allocated among the funds in statute
• Income not currently needed to pay benefits and expenses must be invested in interest-bearing obligations of the U.S. or in obligations guaranteed as to both principal and interest by the U.S.