[1][2][3] It was the result of a partnership between parents (Arthur Sweetser and Ludwik Rajchman) and educators from the Institut Jean-Jacques Rousseau (Adolphe Ferrière and Paul Meyhoffer).

[5] Since its inception, the school's mission has been conceived as educating for peace and the inculcation of humanitarian values such as inclusiveness, respect and inter-cultural understanding.

It describes itself in its website as "resolutely not-for-profit; mankind is the only beneficiary of our work, not corporate shareholders or private equity firms".

The first, bilingual one (Ecole Internationale de Genève – Son premier demi-siècle / International School of Geneva – the first 50 years, Geneva: 1974, 311 pages), edited by René Lejeune (better known as René-François Lejeune), compiles the contributions and eyewitness accounts of various authors, including the historian Robert J. Leach and Ecolint's second director, Marie-Thérèse Maurette.

The nascent school was supported by William Rappard, Rector of the University of Geneva, the neurologist and child psychologist Édouard Claparède,[21] and Sir Arthur Salter, a senior official of the League of Nations.

After occupying rented accommodation on the Rue Charles Bonnet in Geneva's Vieille ville (Old Town), the school finally acquired its own premises in 1929: a historic site known as La Grande Boissière.

[22] The acquisition of this large property was made financially possible by Arthur Sweetser, who personally gave the school thousands of dollars and sought contributions from his network of affluent acquaintances.

These donations included 25,000 U.S. dollars from John D. Rockefeller Jr.[23] Among Ecolint’s notable teachers during the early decades of its existence were Paul Dupuy, formerly doyen (dean) at Paris’ Ecole normale supérieure and defender of Alfred Dreyfus in the late 1890s; the psychoanalyst Charles Baudouin; the philosopher Jeanne Hersch; and the novelist Michel Butor.

[32] It gave rise to a no-confidence referendum in June of that year involving all the school's eligible voters, the outcome of which forced the resignation of both the Governing Board’s Executive Committee and of the Director General.