The Conservative Overseas Bureau officially opened at Conservative Central Office on 1 January 1949, and was put onto a full-time basis from May 1950. Its purpose was to provide hospitality for, and improve links with, overseas visitors, and to ensure that 'the maximum benefit was obtained from the numerous Dominion, Colonial and foreign contacts available to the Party, both centrally and locally'. Its initial responsibilities included:

The Conservative Overseas Bureau Committee acted as the Bureau's executive committee under the leadership of a chairman who was appointed annually by the Party Chairman. Responsibility for putting into effect the Bureau's day-to-day work fell largely to Robert D. Milne, its Secretary from 1949 to 1975. In addition to its primary responsibility of liaising with overseas contacts, a major part of the work of the Bureau was to provide secretariat services to Party groups and committees primarily engaged in foreign or Commonwealth Affairs. These included the Conservative Parliamentary Imperial Affairs Committee (from 1952, renamed the Conservative Parliamentary Commonwealth Affairs Committee, and from 1968 known as the Conservative Foreign Affairs Committee) and the Conservative Commonwealth Council (renamed the Conservative Commonwealth and Overseas Council in 1966). The bulk of the surviving minutes and papers of these groups have survived amongst the papers of the Bureau as a consequence of this.

In 1973, the work of the Conservative Overseas Bureau was absorbed into the newly created International Office at Conservative Central Office. The Office continued to act as a point of contact between the Conservative Party and centre-right parties, individuals and organisations overseas. It also continued to service committees dealing with overseas affairs, particularly the Conservative Commonwealth and Overseas Council, the Foreign Affairs Forum, and the Conservative Foreign and Commonwealth Council.

The establishment of the International Office coincided with the UK's entry into the European Economic Community, and as such, it increasingly provided briefing material and undertook research for Conservative Parliamentarians and members of the Party on matters relating to European affairs just as its predecessor, the Conservative Overseas Bureau, had done in matters of Commonwealth affairs. It also assisted with liaison between the European Parliament and Westminster, and was a point of contact for the Conservative Party's European Parliament groupings - respectively the Conservative European Group (1973-1979), European Democratic Group (1979-1992) and the European People's Party (1992-2009).

Since its inception, the Bureau/International Office engaged with various Christian Democrat and centre-Right forums, primarily within Europe, which emerged in the years after 1945 in response to the increasing international cooperation amongst Social Democrat parties. From the 1970s, the International Office increasingly provided support and encouragement to democratic political parties emerging from former totalitarian regimes, in the Iberian peninsular, Eastern Europe, and outside Europe. After the collapse of the Soviet Union, the Conservative Party, through the International Office's links with groups such as the European Democrat Union (EDU) and International Democrat Union (IDU), gave support and encouragement to the formation of democratic political parties, and sent observers to monitor elections. The Conservative Party was a founding member of both the EDU and the IDU, and from 1983 to 1999 the secretariat of the IDU was based at the International Office.

At the beginning of 1980, the International Office merged with the Foreign Affairs Section of the Research Department (CRD), following CRD's relocation to Smith Square (where Central Office was then based). The newly merged unit was placed under the joint direction of the Party Vice-Chairman for overseas affairs, Sir Anthony Royle, and the Director of the Research Department, Alan Howarth. The main work of the Office was to provide briefing on aspects of foreign policy for Ministers, Conservative Parliamentarians and members of the Party, and to undertake research into international affairs.

By 1992, the four main areas of responsibility of the International Office were outlined as follows: