Ordained a Roman Catholic priest in 1740, Condillac began a lifelong friendship in the same year with the philosopher J.-J. Rousseau, employed by Condillac's elder brother, Jean, as a tutor. Moving to Paris, Condillac became acquainted with the Encyclopaedists, a group of writers led by Denis Diderot. There his position was established in the literary salons by his first book, Essai sur l'origine des connaissances humaines (1746; “Essay on the Origin of Human Knowledge”), and by his second, Traité des systèmes (1749; “Treatise on Systems”). In 1752 he was elected to the Berlin Academy. His Traité des sensations (1754; “Treatise on Sensations”) and Traité des animaux (1755; “Treatise on Animals”) followed, and in 1758 he was appointed tutor to the young prince Ferdinand of Parma. He was elected to the Académie Française in 1768 and later published Le Commerce et le gouvernement considérés relativement l'un à l'autre (1776; “Commerce and Government Considered in Relation to One Another”). Finding the irreligious climate of Parisian intellectual society offensive, he retired to spend his last years at Flux, near Beaugency.

In his works La Logique (1780) and La Langue des calculs (1798; “The Language of Calculation”), Condillac emphasized the importance of language in logical reasoning, stressing the need for a scientifically designed language and for mathematical calculation as its basis. His economic views, which were presented in Le Commerce et le gouvernement, were based on the notion that value depends not on labour but rather on utility. The need for something useful, he argued, gives rise to value, while prices result from the exchange of valued items.

As a philosopher, Condillac gave systematic expression to the views of Locke (Locke, John), previously made fashionable in France by Voltaire. Like Locke, Condillac maintained an empirical sensationalism based on the principle that observations made by sense perception are the foundation for human knowledge. The ideas of the Essai are close to those of Locke, though on certain points Condillac modified Locke's position. In his most significant work, the Traité des sensations, Condillac questioned Locke's doctrine that the senses provide intuitive knowledge. He doubted, for example, that the human eye makes naturally correct judgments about the shapes, sizes, positions, and distances of objects. Examining the knowledge gained by each sense separately, he concluded that all human knowledge is transformed sensation, to the exclusion of any other principle, such as Locke's additional principle of reflection.

Despite Condillac's naturalistic psychology, his statements concerning the nature of religion are consistent with his priestly vocation. He maintained a belief in the reality of the soul, which did not conflict, in his view, with the opening words of the Essai: “Whether we rise to heaven, or descend to the abyss, we never get outside ourselves—it is always our own thoughts that we perceive.” This doctrine became the foundation of the French philosophical movement known as Idéologie (Ideology) and was taught for more than 50 years in French schools.