In 1898, he entered the École Normale Supérieure, where he studied history and won a travelling scholarship which enabled him to visit Russia.

When the First World War broke out, Thomas served in a territorial regiment of the French Army for a few weeks after which he was summoned to Paris and placed in control of the railway services, acting as a link between the General Staff and the Ministry of Public Works.

In November 1919, during the inaugural Session of the International Labour Conference in Washington, at which he was not present, the Governing Body of the ILO chose him to be the Director of the Office.

In a few years, he created, out of a small group of officials housed in a private residence in London, an international institution with a staff of 400 and a building of its own in Geneva.

From 1920 onwards, the ILO launched an ambitious programme of publications, which included the Official Bulletin, the monthly International Labour Review and various other periodicals and newspapers.

Opposition between national governments, organised labour and employer representative bodies soon began to develop, and the optimism that had prevailed immediately following the end of the war gave way to doubt and pessimism.

In 1921, the French government took the position that the ILO was not competent to deal with agricultural matters and the Permanent Court of International Justice was requested to give an advisory opinion on the question.

The Court found that the competence of the ILO did extend to international regulation of the conditions of labour of persons in agriculture, rejecting a restrictive interpretation of the Constitution.

In 1926, an important innovation was introduced when the International Labour Conference set up a supervisory system on the application of its standards, which still exists today.

He was a great believer in the "policy of presence", and he spent a good deal of time travelling in order to seek support for the objectives and functions of the Organisation.