Momentum for an international organisation for like-minded Christian Democrat, centre-right parties around the world grew once non-European nations, including Japan, Australia and New Zealand, began to take up associate membership of the European Democrat Union (EDU). In 1982, inter-party meetings between the EDU and the newly-established Pacific Democrat Union (PDU) resulted in the creation of an International Democrat Union. The IDU was formally founded on 24 June 1983 in London. Its founding declaration was signed by 19 leaders from moderate and centre-right parties in Austria, Australia, Canada, Cyprus, Denmark, Finland, France, Germany, Greece, Japan, New Zealand, Norway, Portugal, Spain, Sweden, the United Kingdom, and the United States.

The IDU's founding principles were similar to those of the EDU, with an emphasis upon supporting 'the free, open and democratic society, the socially-orientated market economy, the rule of law, social justice, the natural role of the family, and a society where conditions are created to enable everyone to reach their full potential and to carry out their responsibilities to their fellow men and women'.

An Executive Committee, composed of the Chairman and Executive Secretaries of the IDU, EDU and PDU, and the Treasurers and Deputy Chairman and Vice-Chairmen of the IDU, oversees the activities of the IDU and establishes any standing committees and ad-hoc working groups – the most prominent of which was the Standing Committee on Foreign Affairs. A Party Leaders Conference, composed of the IDU's governing body, would meet every two years to consider membership applications and policy directives, and to elect IDU officers.

Membership of the IDU is open to all full members of the EDU, PDU and CDU, as well as parties who adhere to the IDU's Declaration of Principles. Associate membership is also offered to parties and organisations who want consultative status. Like the EDU, fact-finding missions were conducted by the IDU to strengthen the organisation's contacts in particular countries. A youth wing – the International Young Democrat Union – was established in 1991, and an International Women's Democrat Union was founded in 1990 (see COB 7/12).

The IDU operated closely with the Conservative Party, to the extent that between 1983 and 1999, the day-to-day operation of the IDU were run from the Party's International Office at Conservative Central Office, where the IDU secretariat was based. The Head of the International Office also held the dual role of Secretary General of the IDU. As a consequence of this close relationship between the two organisations, a large number of the IDU's papers have survived within the papers of the International Office.

The IDU continues to exist as the umbrella organisation for the European Democrat Union (see COB 7/4), Caribbean Democrat Union (see COB 7/1) and Pacific Democrat Union (see COB 7/14).