Moissan came from a poor background in Paris, France. He was the son of a railroad worker and was apprenticed to a pharmacist before studying chemistry under Edmond Frémy at the Muséum National d'Histoire Naturelle, Paris (1872). From 1880 he worked at the Ecole Supérieure de Pharmacie, being elected to the chair of toxicology in 1886 and the chair of inorganic chemistry in 1889. In the next year he became professor of chemistry at the University of Paris.

Moissan began studying fluorine compounds in 1884 and in 1886 succeeded in isolating fluorine gas by electrolyzing a solution of potassium fluoride in hydrofluoric acid, the whole process being contained in platinum. He received the Nobel Prize for chemistry for this work in 1906.

He also worked on synthetic diamonds. He was impressed by the discovery of tiny diamonds in some meteorites and concluded from this that if the conditions undergone by these in space could be reproduced in the laboratory it would be possible to convert carbon into diamond. He therefore put iron and carbon into a crucible, heated it in an electric furnace, and while white hot cooled it rapidly by plunging it into liquid. In theory, he felt that the cooling should exert sufficient pressure on the carbon to turn it into diamond. He claimed to have succeeded in producing artificial diamonds but there was a suggestion that one of his assistants had smuggled tiny diamonds into the mixture at the beginning of the experiment. Moissan did, however, use his electric furnace for important work in preparing metal nitrides, borides, and carbides, and in extracting a number of less common metallic elements, such as molybdenum, tantalum, and niobium.