Through the etymology, number of words (Illiez, Vièze, Nant) lets suppose that the dialect of Valley of Illiez is of immediate Celtic origin, and that the first populating of the valley of Illiez already took place at the time of the Celtic penetration in Gaule, is in the neighbourhood of the 6th century BC. Moreover, at the time of the Roman conquest, Tête-du-Lac (Caput Lacense), in popular language Capo-lay, where from came the modern name of Chablais, was lived by Nantuates. This name, stemming from the Celtic "nanto", means the valley (it indicated afterward the brook, the river of the valley).
A faith still strongly anchored at the beginning of the 20th century gave the origin of the inhabitants of the Val d' Illiez to Roman soldiers. These, having escaped in martyr of the Thebean legion, massacred on order of Maximien between the year 285 and the year 302, would have taken refuge on the heights and would have the first cultivated the Valley of Illiez. However this faith leans on no proof.
To Romain who remained in the Valais during four hundred years, succeeded Burgondes, by the middle of the 5th century. The Valais was then a part of the empire of the Francs (534 ), whom supremacy had to remain until the creation of the second kingdom of Burgundy by Rodolphe 1st in 888. It is during this period that the small country of Tête-du-Lac to which was connected Val d' Illiez was formed. It became under the supremacy of the house of Savoy, but we don't know neither the origin nor the date of this acquisition.
From 1036 to 1536 in the middle Ages, the principle of the division of powers existed hardly. That is why it was so difficult to classify their representatives and to determine their attributions. Some documents give evidence that there was a spot called " The Court " a tower, what seems to have been a castle in the Middle Age, a headquarter of the lordship of Valley-d'Illiez and Champéry (Troistorrents being a part of the one of Monthey). This tower was situated in a place from where we could best watch the passage of the pilgrims who went from the Chartreuse of Mélan-Taninges or from Saint-Jean d' Aulps to Saint-Maurice or vice versa. It seems to be towards the end of the 16th century that the lordship disappeared, the abbey of Saint-Maurice having widened its jurisdiction on the entire valley.
From 1536 to 1798, the community of the Valley d'Illiez already enjoyed some freedom and liberties. The occupation of the Haut-Valais confirmed them, although the governor charged them since 1690. The very defective way the justice was returned had annoyed the population of the Bas-Valais. From 1790, people began to get angry, an uprising in which Pierre-Maurice Rey-Bellet aka "Gros-Bellet" participated in a very active way to bring the Bas-Valais to the independence in 1798. In its memory, a statue is set up on the place of the village.
In 1803 begun the first steps. The reason was that, in the previous century, Champéry had a religious chancellorship. On 23.11.1839, the state council pronounces the separation of the municipalities of Illiez and Champéry. Note that the sector of Tière (between the torrents of Soi and Tière), was given to Champéry, but finally opted according to its right for Val-d'Illiez, what brought a modification of the limits and, therefore, Val-d'Illiez kept the totality of the north hillside of the Dents-du-Midi.