Monnet firmly believed that the only path to an Allied victory lay in combining the war efforts of Britain and France, and he reflected on a concept that would coordinate war resources. In 1914, a friend of the family, Fernand Benon, arranged for young Monnet to meet French Premier René Viviani in Bordeaux to talk about this issue; Monnet managed to convince the French government to agree with him, in principle.

At the Paris Peace Conference, Monnet was an assistant to the French minister of commerce and industry, Étienne Clémentel, who proposed a “new economic order” based on European cooperation. The scheme was officially rejected by the Allies in April 1919.

Due to his contributions to the war effort, Monnet, at the age of thirty-one, was named Deputy Secretary-General of the League of Nations by French premier Georges Clemenceau and British statesman Arthur Balfour, upon the League’s creation in 1919.

Soon disillusioned with the League because of its laborious and unanimous decision-making processes, Monnet resigned in 1923 and devoted himself to managing the cognac family business, which was experiencing difficulties. In 1925, Monnet moved to America to accept a partnership in Blair & Co., a New York bank which merged with Bank of America in 1929, forming Bancamerica-Blair Corporation which was owned by Transamerica Corporation. He returned to international politics and, as an international financier, proved to be instrumental to the economic recovery of several Central and Eastern Europe nations. He helped stabilise the Polish złoty in 1927 and the Romanian leu in 1928. In November 1932, the Chinese Minister of Finance invited Jean Monnet to act as chairman of an east–west non-political committee in China for the development of the Chinese economy where he lived until 1936. During his time in China, Monnet’s task of partnering Chinese capital with foreign companies led to the formal inauguration of the Chinese Development Finance Corporation (CDFC) as well as the reorganization of the Chinese railroads.

In August 1929, during a dinner party in Paris, the 41-year-old Monnet met 22-year-old Italian painter Silvia Giannini (17 August 1907 – 22 August 1982) who had recently married Francisco Giannini, an employee of Monnet when he was a representative in Italy. In April 1931, Silvia gave birth to a daughter, Anna, whose legal father was Giannini.

While in Washington, Monnet and his family lived in a comfortable house built in 1934 at 2415 Foxhall Road NW, which was later home to Adlai Stevenson III and James Baker. He worked from an office of the British Mission at the Willard Hotel.

Since divorce wasn’t allowed in most European countries, Silvia and Jean Monnet met in Moscow, as it was possible there to obtain citizenship and residence qualifications rapidly, and to divorce and remarry at once. In 1934, he returned from China via the Trans-Siberian railway, she from Switzerland. He arranged for Silvia to obtain Soviet citizenship; she immediately divorced her husband and married Jean Monnet. The idea for the Moscow marriage came from Dr. Ludwik Rajchman, whom Monnet had met during his time at the League of Nations (Rajchman was connected to the Soviet Ambassador to China, Dmitrij Bogomołow). It seems that the American and French ambassadors in Moscow, William Bullitt and Charles Alphand, also played a role. The custody of Anna was a problem; in 1935 Silvia took refuge with Anna in the Soviet consulate in Shanghai, where they were living at the time because Francisco Giannini was trying to obtain custody of the child. The legal battle was decided in favour of Silvia in 1937 in New York, but the ruling wasn’t recognized by some other countries.

In 1935, when Monnet was still in Shanghai, he became a business partner of George Murnane (a former colleague of Monnet at Transamerica) in Monnet, Murnane & Co. Murnane was connected to the Wallenberg family in Sweden, the Bosch family in Germany, the Solvays and Boëls in Belgium, and John Foster Dulles, André Meyer, and the Rockefeller family in the United States. He was considered among the most connected persons of his time.

In December 1939, Monnet was sent to London to oversee the collectivization of the British and French war industries. In his memoirs, Monnet claims that his influence inspired Charles de Gaulle and Winston Churchill to agree on an Anglo-French union, ostensibly in an attempt to rival the alliance between Germany and Italy, an event, however, not mentioned in de Gaulle’s or Churchill’s memoirs on the war.

In August 1940, Monnet resigned as head of the Inter-Allied Commission and was sent to the United States by the British Government, as a member of the British Supply Council, to ask for rapid and massive increases in the flow of American weapons so Britain could turn the tide against Germany. Soon after his arrival in Washington, D.C., he became an advisor to President Franklin D. Roosevelt. Convinced that America could serve as “the great arsenal of democracy”, he persuaded the President to launch a massive arms production program, both as an economic stimulus and to supply the Allies with military resources. Unlike De Gaulle, Monnet was popular with the Americans and the English. A co-worker of his claims that sometimes he would help Churchill to write a message to Roosevelt and then on the next day help Roosevelt to compose the response. According to Edward R. Kantowicz, Monnet was impressed by the American organizational energy and saw cooperation with the new superpower as Europe’s only chance to reorganize and recover itself. In 1941, Roosevelt, with Churchill’s agreement, launched the Victory Program, which represented the involvement of the United States in the war effort. After the war, British economist John Maynard Keynes stated that Monnet, through his coordination work, had probably shortened World War II by a year.

In 1941 Monnet and Silvia had another daughter, Marianne. The Monnet family returned to France in 1945 and, after the death of Francisco Giannini in 1974, the couple married canonically in the cathedral of Lourdes. Silvia Monnet was very important to her husband throughout their forty-five years of marriage, according to Louis Joxe, Monnet “would spend hours writing to his wife, whose opinion mattered more to him than that of anyone else”.

In 1943, Monnet became a member of the National Liberation Committee, De Gaulle’s French government-in-exile, stationed in Algiers, being designated Commissaire à l’Armement (Minister of Armaments). During a meeting on 5 August of that year, Monnet declared to the Committee:

Following World War II, France was in severe need of reconstruction and completely dependent on coal from Germany’s main remaining coal-mining areas, the Ruhr and the Saar. (The German coal fields in Upper Silesia had been handed over to Polish administration by the Allies in 1945, see Oder-Neisse line.)

In 1945, Monnet proposed the Monnet Plan, also known as the “Theory of l’Engrenage” (not to be confused with the Schuman plan). It included taking control of the remaining German coal-producing areas and redirecting the production away from the German industry and into the French, thus permanently weakening Germany and raising the French economy considerably above its pre-war levels. The plan was adopted by Charles de Gaulle in early 1946.

The Jean Monnet House is located in Houjarray, a hamlet of Bazoches-sur-Guyonne, Yvelines, 80 kilometres (50 miles) outside Paris. This old farm became Jean Monnet’s property in 1945, upon his return to France. It was there that Jean Monnet and his advisors, in the last days of April 1950, drew up the historic declaration that Robert Schuman used to address Europe on 9 May 1950, proposing the creation of the European Coal and Steel Community. In the house, Robert Schuman, Walter Hallstein, Paul-Henri Spaak, Konrad Adenauer, René Pleven, Helmut Schmidt, and many others exchanged their views with Jean Monnet on Europe’s common future. On Sundays, he had friends passing by come to his house; among them were Dwight Eisenhower, George Ball, and Edward Heath. Monnet liked fireside conversations with journalists such as Walter Lippman, Hubert Beuve-Méry, or his neighbour Pierre Viansson-Ponté. This house was also where Jean Monnet died on 16 March 1979.

In 1947 France removed the Saar from Germany, with U.S. support, and turned it into the Saar Protectorate, which was essentially a French vassal state. The area returned to German political administration in 1957 (economic reunification would take many years longer), but France retained the right to mine from its coal mines until 1981. (See: The Europeanisation of the Saarland)

When tensions between France and Germany rose over the control of the then vital coal and steel industries, Monnet and his associates conceived the idea of a European Community. On 9 May 1950, with the agreement of Chancellor Konrad Adenauer of West Germany, French Minister of Foreign Affairs Robert Schuman made a declaration in the name of the French government. This landmark pronouncement, prepared by Monnet for Schuman and known as the Schuman Declaration, proposed integration of the French and German coal and steel industries under joint control, a so-called High Authority, open to the other countries of Europe. Schuman declared:

When Germany agreed to join the European Coal and Steel Community according to the Schuman Plan in 1951, the ongoing dismantling of German industry was halted and some of the restrictions on German industrial output were lifted. West Germany joined the ECSC, alongside Italy, Belgium, Luxembourg and the Netherlands, while Britain refused, on grounds of national sovereignty.

In 1952, Jean Monnet became the first president of the High Authority and with the opening of the common market for coal under the ECSC in 1953, the last civilian production limitations placed on German industry were lifted, and the role of the IAR was taken over by the ECSC.

In 1953 Monnet was awarded the Karlspreis by the city of Aachen in recognition of his achievements.

In 1955, Monnet founded the Action Committee for the United States of Europe in order to revive European construction following the failure of the European Defence Community (EDC). It brought political parties and European trade unions together to become a driving force behind the initiatives which laid the foundation for the European Union as it eventually emerged: first, the European Economic Community (EEC) (1958) (known commonly as the “Common Market”), which was established by the Treaty of Rome of 1957; later the European Communities (1967) with its corresponding bodies, the European Commission and the European Council of Ministers, British membership of the Communities (1973), the European Council (1974), the European Monetary System (1979), and a directly elected European Parliament (1979). This process reflected Monnet’s belief in a gradualist approach for constructing European unity.

The Jean Monnet Foundation for Europe in Lausanne supports initiatives dedicated to the construction of European unity. Its origins date from a meeting between Jean Monnet and Henri Rieben in 1955. It is located on the campus of the University of Lausanne and houses the personal archives of Jean Monnet as well as those of Robert Schuman, Robert Marjolin, François Fontaine, Jacques Van Helmont, Paolo Emilio Taviani, Robert Triffin, and the Earl of Perth.

On 6 December 1963, he was presented with the Presidential Medal of Freedom, with Special Distinction, by United States President Lyndon Johnson. The decision had been made by President Kennedy before his assassination.

In 1972, Queen Elizabeth II made him an honorary Companion of Honour.

Monnet resigned and ended the Committee’s activity on 9 May 1975, the 25th anniversary of the Schuman Declaration.

Writing about his life and the principles which drove his action was a longstanding project which Monnet delayed for years as he gave higher priority to his other projects. In the early 1970s, François Fontaine was instrumental in bringing the endeavor to fruition and drafted much of the text, even as Monnet retained ultimate control. The Memoirs were published by Fayard in 1976, and an English translation by Richard Mayne by Doubleday in 1978.

He was the first to be bestowed Honorary Citizen of Europe by the European Council of the European Union, for extraordinary work to promote European cooperation on 2 April 1976. Following this, he became the first person then living to be pictured on a Federal Republic of Germany stamp who was not a German head of state.

On 16 March 1979, Jean Monnet died at the age of 90 in his home in Houjarray, Bazoches-sur-Guyonne.

The European Parliament acquired the house in 1982 and entrusted its restoration, management, and organization to the Jean Monnet Association. A multimedia conference room was added in 2000. The Jean Monnet Association organizes about 250 conferences on European history and current events each year.

In 1988, on the hundredth anniversary of his birth, he was reburied in the Panthéon in Paris in a ceremony attended by numerous European heads of state and government.

Several prizes also seek to honour Jean Monnet’s life and achievements. The Jean Monnet Prize for European Literature [fr], set up in 1995 by the department of Charente, rewards European authors for books written in, or translated to, French. The Jean Monnet Prize for European Integration, given by EuropeanConstitution.eu, a French association, rewards projects contributing to the promotion of European integration. Several universities and research centres also award prizes named after Jean Monnet.

In April 2011, to commemorate the 60th anniversary of the signing of the Treaty of Paris, a new documentary, “Jean Monnet: Father of Europe” was produced. The documentary includes interviews with colleagues of Monnet such as Georges Berthoin [fr], Max Kohnstamm and Jacques-René Rabier, as well as former member of the European Court of Justice David A.O. Edward of the United Kingdom.

In September 2016, Prof. Dr Sebastian Bersick was awarded a Jean Monnet Chair by the European Commission under the Erasmus+ Programme. The funding enables the Ruhr-University Bochum (RUB), the Faculty of East Asian Studies, and the Department of International Political Economy of East Asia to intensify teaching and research activities and to organize new initiatives.