Born in Laon, France, in a cultured yet modest family, Méchain studied mathematics and physics, but he was obliged to give up. He was remarked by the astronomer Jerôme Lalande (1732-1807), who hired him as an assistant, and he became friends with Charles Messier (1730-1817). Méchain assiduously observed the sky and discovered a large number of nebulous objects that were inserted by Messier in his famous catalog. He also discovered 14 comets between 1779 and 1799. He was elected to the Academy of Sciences in 1782; in 1788 the Academy put him in charge of its ephemeris, the Connaissance des temps. He administered the Paris Observatory from 1800 to his death. The Convention required him to measure, with Jean-Baptiste Delambre (1749-1822), the length of the Paris meridian from Dunkirk to Barcelona, to obtain the value of the meter, which was defined as the 10 millionth of a quarter of the terrestrial meridian. Méchain went from Rodez to Barcelona, while Delambre was responsible for the northern part. These measurements, undertaken despite many difficulties of between 1792 and 1798, were very good, but Méchain was much affected by an abnormal result obtained in Barcelona. He returned there in 1804, but died in Castellon de la Plana, probably of malaria, without solving the problem.