Marcet was born at Geneva, and received his school education there. He went in 1794 to Edinburgh university, where he became M.D. on 24 June 1797, writing a thesis on diabetes, printed at Edinburgh in the same year. He then took a house in London, worked as assistant physician at Cary Street Dispensary, and was admitted a licentiate of the Royal College of Physicians on 25 June 1799. He then became a physician at Finsbury Dispensary, and at Guy's Hospital on 18 April 1804. He lectured there on chemistry between 1805 and 1819.