On his arrival in Paris in January 1919, Butler prepared the first draft of the Preamble to the Constitution which resulted in the setting up by the Peace Conference, on 31 January 1919, of the Commission on International Labour Legislation. On 11 April 1919 the Peace Conference accepted the proposals of the Commission and set up an Organising Committee to prepare for the first International Labour Conference, which appointed Butler as its Secretary. The first International Labour Conference met in Washington on 29 October 1919 and appointed Butler as its Secretary-General.
In Washington, it was Butler's duty as Secretary-General of the Conference to put into practice for the first time the principles embodied in the Constitution of the Organisation. Under Butler's leadership the Conference faced and solved innumerable problems of procedural technique for which experience afforded no guide. At the second session of the Governing Body in Paris in January 1920, Albert Thomas was appointed Director of the ILO, and his first act was to appoint Butler Deputy Director.
As Deputy Director of the Organisation, Butler's special assignment was internal organisation, administration and finance. He concerned himself specially with the problems of personnel and recruitment, always emphasising the importance of creating a genuinely international staff. Butler was convinced that it was possible to obtain loyal cooperation and a high standard of performance from an international staff, and he spared no effort to ensure that, in spite of the enormous difficulties presented by different languages, different systems and methods of working, and even different ways of thinking, the group of newly recruited officials should be as rapidly as possible transformed into a unified, homogeneous and loyal staff of international civil servants.
When, in 1932, Butler was appointed Director of the International Labour Office after the sudden death of Thomas, it was clear that troubled times were ahead for the Organisation. In the economic sphere, the world was under the cloud of the Great Depression. Governments restricted trade and foreign exchanges in order to safeguard their balance of payments. Unemployment mounted steadily and financial, economic and social security were progressively undermined. Political developments were no better: the Disarmament Conference was failing to reach an agreement, events in Manchuria showed that the authority of the League of Nations could be flouted with impunity, and in many countries acute political unrest verged on civil war. Butler took steps to buttress the Organisation in case another world war should break out. To his mind, the greatest single reinforcement that could be looked for was the entrance of the United States into the Organisation, which it did in 1934.
Butler also worked to enable closer and more effective participation of overseas, or extra-European, countries in the work of the Organisation. At the 1934 session of the Conference, after a delay of twelve years, the Governing Body included seven extra-European countries among the sixteen Governments represented, with corresponding increases in extra-European representation in both the Employers' and Workers' groups. Butler also took steps to intensify direct relations between the Office and overseas countries in order to better understand their conditions and needs and to provide them with assistance. Officials were sent on mission to Latin America, Asia and the Middle East. An Overseas Section was set up in the Office to ensure that greater attention should be paid to the special problems of overseas countries. In January 1936, the first Regional Conference was held in Santiago, Chile.
During Butler's term as Director of the Organisation, efforts were made to look at the labour and industrial problems of individual industries. The first such meeting was the International Textile Conference held in Washington in 1937. Other technical conferences followed on hours of work in the coal-mining, chemical and other industries. These technical tripartite conferences were the precursors of the industrial committees set up after the war.
In 1938 Butler resigned in order to accept an invitation to be Warden of the newly established Nuffield College at Oxford. He then went on to become wartime regional commissioner in England, British Minister in charge of information services in Washington, Chairman of the European League for Economic Cooperation. Throughout that time, Butler maintained a keen interest in the ILO. His appointment, in 1950, as a member of the Fact-finding and Conciliation Commission on Freedom of Association brought him into official relationship with the Organisation once again. Harold Butler died in 1951 at the age of sixty-seven.