Brown-Sé quard was born in Port Louis, Mauritius, and studied medicine in Paris, graduating in 1846. He was professor of physiology and pathology at Harvard (1864–68) and in 1878 succeeded Claude Bernard as professor of experimental medicine at the Collège de France. The intervening years were spent in a variety of posts in New York, London, and Paris. He is perhaps best known for his work on the adrenal gland. In his experiments on hormonal secretions, he demonstrated the connection between excision of the adrenal glands and Addison's disease.

Continuing the work of Galen on dissection of the spinal cord, he discovered the Brown-Séquard syndrome (crossed hemiplegia), a condition of motor nerve paralysis resulting from the lesion of one side of the spinal cord. This produces an absence of sensation on the opposite side of the body to the nerve paralysis. Brown-Séquard also investigated the possibility of prolonging human life by the use of extracts prepared from the testes of sheep. The majority of his research findings were published as papers in the Archives de physiologie, of which he was one of the founders.