Stas was born in Leuven and trained initially as a physician. He later switched to chemistry and worked at the École Polytechnique in Paris under the direction of Jean-Baptiste Dumas, with whom he established the atomic weight of carbon.
In 1840, he was appointed professor at the Royal Military School in Brussels. He acquired international fame by establishing the atomic weights of the elements more accurately than had ever been done before, using oxygen = 16 as a standard. His results disproved the hypothesis of the English physicist William Prout that all atomic weights must be integral multiples of that of hydrogen. Stas thus laid the foundations for the formation of the periodic system of elements by (among others) Dmitri Mendeleev.
Stas retired in 1869 because of problems with his voice caused by a throat ailment. He became commissioner of the mint, but resigned in 1872 because he disagreed with the government's monetary policy. Jean Stas died in Brussels.