His father (a lawyer) later discouraged him from continuing as a Doctor and René then had a period of time where he took long walks in the country, danced, studied Greek and wrote poetry. However, in 1799 he returned to study. Laennec studied Medicine at the University of Paris under several famous Physicians, including Dupuytren and Jean-Nicolas Corvisart-Desmarets. There he was trained to use sound as a diagnostic aid. Corvisart advocated the re-introduction of percussion during the French Revolution.

He coined the term melanoma and described metastases of melanoma to the lungs. In 1804, while still a medical student, he was the first person to lecture on melanoma. This lecture was subsequently published in 1805. Laennec actually used the term 'melanose,' which he derived from the Greek (mela, melan) for "black." Over the years, there were bitter exchanges between Laennec and Dupuytren, the latter objecting that there was no mention of his work in this area and his role in its discovery.

René Laennec wrote the classic treatise De l'Auscultation Médiate, published in August 1819 The preface reads:

The stethoscope quickly gained popularity as De l'Auscultation Médiate was translated and distributed across France, England, Italy and Germany in the early 1820s. However, not all doctors readily embraced the new stethoscope. Although the New England Journal of Medicine reported the invention of the stethoscope two years later in 1821, as late as 1885, a professor of Medicine stated, "He that hath ears to hear, let him use his ears and not a stethoscope." Even the founder of the American Heart Association, L. A. Connor (1866–1950) carried a silk handkerchief with him to place on the wall of the chest for ear auscultation.

He became a lecturer at the Collège de France in 1822 and professor of Medicine in 1823. His final appointments were that of head of the medical clinic at the Hôpital de la Charité and professor at the Collège de France. He died of tuberculosis in 1826 at the age of 45.

The modern binaural stethoscope with two ear pieces was invented in 1851 by Arthur Leared. George Cammann perfected the design of the instrument for commercial production in 1852, which has become the standard ever since.

Laennec "was intensely religious and was a devout Catholic all his life". He was noted as a very kind man and his charity to the poor became proverbial. Austin Flint, the 1884 President of the American Medical Association, said that "Laennec's life affords a striking instance among others disproving the vulgar error that the pursuit of science is unfavourable to religious faith."

He was the subject of a 1949 French film Doctor Laennec in which he was played by Pierre Blanchar.

On the exterior wall of the "Hôpital Necker – Enfants Malades", where Laennec wrote Mediate auscultation, near the entrance of the hospital in 149, Rue de Sèvres, there is a marble memorial tablet with an engraved portrait of Laennec and this inscription: "Dans cet hôpital Laennec découvrit l'auscultation. 1781–1826". Some of the oldest buildings of the hospital can be seen on the same front of this large and modern medical area.

He also studied tuberculosis. Coincidentally, his nephew, Mériadec Laennec, is said to have diagnosed tuberculosis in Laennec using Laennec’s stethoscope. Laennec wrote A Treatise on the Disease of the Chest, in which he focused on diseases of the chest such as Phthisis pulmonalis and diagnostics such as Pectoriloquy. He discussed the symptoms of Phthisis pulmonalis and what parts of the body it affects. It was written in an academic manner for learning purposes.