Laënnec was born in Quimper, Brittany, the son of an unsuccessful lawyer. After his mother's death he was brought up mainly by his uncle G. F. Laënnec, professor of medicine at the University of Nantes. He was a pupil of Jean Corvisart at the Charité in Paris, qualifying as a doctor in 1804. He worked at a number of hospitals before becoming in 1814 physician-in-chief at the Hôpital Necker, where he remained until 1822. Then he was appointed professor of medicine at the Collège de France. In 1826 he was forced to retire to Brittany where he died of consumption.

In 1819 Laënnec published one of the classic texts of modern medicine, De l'auscultation médiate (On Mediate Auscultation). It advanced the work of Leopold Auenbrugger in describing sounds detected within the body and the various diseases and anatomical defects they were related to. The work is, however, best known for its description of the situation leading to his invention of the stethoscope.

In 1816 he was consulted by a young woman with heart trouble whose age and sex inhibited him from examining her by his usual method, namely, placing his ear on her breasts. Instead, Laënnec tightly rolled a sheaf of paper and placed one end over the heart and the other to his ear. He was surprised and pleased to find that the heartbeat could be heard far more clearly and distinctly than before. The work became widely known and was translated into English in 1821 and only two years later was published in America, where it was vigorously promoted by Austin Flint. The stethoscope itself, in improved flexible versions, rapidly became a standard part of the physician's equipment.