He studied medicine at the University of Paris under Jean-Étienne Dominique Esquirol (1772–1840), and while a student worked as an intern at the Charenton mental institution.

In 1840 Baillarger was the first physician to discover that the cerebral cortex was divided into six layers of alternate white and grey laminae.

[2][3] In the field of psychiatry, Baillarger did research on the involuntary nature of hallucinations and the dynamics of the hypnagogic state (the intermediary stage between sleep and wakefulness).

In 1854 he provided a description of a psychiatric disorder involving both manic and depressive episodes in the same individual, a condition that he referred to as folie à double forme (dual-form insanity).

Unbeknownst to him at the time, another French psychiatrist, Jean-Pierre Falret (1794–1870), had described fundamentally the same condition (with a number of salient differences) in an article prior to Baillarger's findings.