During the First World War worked in London as a delegate of the French government to coordinate economic cooperation between the allies. In 1919, he participated in the creation of the League of Nations, where he was Deputy Secretary-General.
During World War II he chaired the Coordinating Committee for Franco-British pooling of resources allies. Even in 1943, in Algiers, with his country occupied by German troops, Monnet and his ideals Europeans raised:
After the war, was appointed Commissioner of the Plan of reconstruction and economic recovery of France.
In 1950, he suggested to French Foreign Minister Robert Schuman, the idea of integrating the French and German production of coal and steel. The formal proposal of the Minister, the Schuman Plan, eventually led to the establishment of the European Coal and Steel Community in 1952. The first serious step in the path of European unity. Monnet was elected president of the High Authority who led the ECSC.
The veto of the French Parliament in 1954 marked the first major failure in the process of unity and led to Monnet resigned to be reelected as head of the ECSC.