The two men who started JMI could hardly have come from more different backgrounds. Cuvelier had grown up in a Brussels family that could afford violin lessons at the Royal Conservatory and law school at ULB. Rene Nicoly, born in Avon in 1907, was the son of a manservant and a chambermaid. What gave Nicoly his music was an accident of his parents' employment - they worked for the Parisian music publisher Jacques Durand, who taught the boy clarinet himself and later gave him a job in the publishing house. By the late 1930s Nicoly was running music-education programs in Parisian high schools, working hard to give young people from every walk of life serious musical experiences. The two met in autumn 1941, when Nicoly visited Brussels and discovered Cuvelier was organizing similar concerts for Belgian youth. The war made any larger collaboration impossible. They kept in touch, and on July 17, 1945, with both their countries newly freed, they joined forces and announced the formation of an international youth music movement.

The first general meeting drew delegates from seven countries to Brussels on May 16 and 17, 1946. Luxembourg was the first to formally join in 1947; the Netherlands followed in 1948, then Austria, Portugal, and Switzerland in 1949. Canada came in 1950 as the first non-European member. The early years built outward through Europe, then jumped to Latin America - Cuba, Haiti, and Brazil in 1953 - and eventually to Asia. In 1957 the organization made an unusual decision: only politically independent member organizations could join. In an era when state-sponsored youth movements were a defining feature of both Cold War sides, JMI declared that its sections had to be free of government control and led with real participation by young people themselves. The age limit was set at thirty. The same year, the first transcontinental tours began. By the 1990s the organization had reached fifty member states; today it operates in more than sixty countries and organizes more than 40,000 musical events a year, reaching roughly six million young people.

Some of the most striking moments in JMI's history have happened around walls. The first International Orchestra performed at the fourth conference in Scheveningen in 1949, conducted by Igor Markevitch. In 1969, the organization established a summer music camp in Groznjan, a small medieval hill town in what was then Yugoslav Istria. The town had been emptying out in the postwar decades; painters and sculptors had begun restoring it in 1965, and the arrival of JMI's young musicians brought the houses back to life. Groznjan is still known in Croatia as the City of Artists, and JMI music courses still run there from June through September. But the most symbolically charged performance came in 1987, when the Jeunesses Musicales World Orchestra performed Benjamin Britten's War Requiem on both sides of the Berlin Wall, accompanied by a boys' choir from the United States and the Wiener Jeunesses Choir. Britten had written the Requiem in 1962 for the consecration of Coventry Cathedral, rebuilt after the city had been destroyed in the war. To play it across a wall that still divided a continent, by an orchestra that included Eastern Bloc members admitted years before formal politics had allowed it, was the kind of gesture the organization had been preparing to make for forty years.

Some of the programs that have grown out of JMI have themselves become institutions. The Ethno project began in 1990 as an initiative of JM Sweden, gathering young folk musicians from across the world for summer camps where they teach each other traditional songs by ear - no scores, no recordings, just the slow patient transmission of one person's melody to another. The European Youth Forum named it good practice in non-formal education. Ethno camps now run annually in Belgium, Croatia, Cyprus, Denmark, Estonia, Slovenia, Sweden, Norway, France, Portugal, Germany, Bosnia and Herzegovina, India, Australia, and Uganda. A separate initiative, Music Crossroads, was launched in 1995 to bring the same approach to southern Africa - Malawi, Mozambique, Tanzania, Zambia, and Zimbabwe - where independent national organizations now run their own festivals and band tours. In 2013, JMI helped launch Music against Child Labour, a partnership with the International Labour Organization that drew endorsements from conductors and soloists including Claudio Abbado, Daniel Barenboim, and Jose Antonio Abreu of El Sistema.

Cuvelier did not live to see most of it. In 1958 he suffered a heart attack in Tashkent, where he had traveled to accompany Queen Elisabeth of Belgium to the Tchaikovsky Competition. He was warned to rest. He kept working. He was found dead in September 1959 in a hotel room in Venice, where he had gone to attend a meeting of the International Music Council. Nicoly continued until 1971, by then also serving as Director of the Paris Opera. The organization passed through other secretary-generals - Paul Willems through the 1960s and early 1970s, Hadelin Donnet through the 1980s - and through other logos. The original 1952 emblem, a musical note on a globe, was replaced in the 1990s by a red oval with a black stem rising from it. Christoph Platen, defending the new symbol, called it a deliberate evocation of contrast carried into wholeness - red and black, familiar and exotic, music as a way across political, ethnic, and cultural divides. The Palace of Fine Arts on the Rue Ravenstein, where Cuvelier first dreamed up the idea as a young man walking the construction beams in 1928, is still where the organization keeps its headquarters today.