Hélyot was born at Paris in January 1660, supposedly of English ancestry, and christened Pierre at his birth.
After spending his youth in study, he entered, in his twenty-fourth year, the friary of the Third Order Regular of St. Francis, founded in Picpus – now part of Paris – by his uncle, Jérôme Hélyot, a canon regular of the Order of the Holy Sepulcher.
Both in Italy and France he was engaged in collecting materials for his great work, which occupied him for about twenty-five years.
It was titled L'Histoire des ordres monastiques, religieux et militaires, et des congregations séculières de l'un et de l'autre sexe, qui ont été établis jusqu'à présent (The History of the Religious and Military Monastic Orders, and of the Secular Congregations of both Sexes, which have been established up to the Present Day).
The material was arranged in an alphabetical dictionary form by M. L. Badiche, for inclusion in Migne's Encyclopédie théologique, under the title "Dictionnaire des ordres religieux" (5 vols., 1858).