Stas, who was born at Louvain in Belgium, trained initially as a physician. He later switched to chemistry, serving as assistant to Jean Dumas before being appointed to the chair of chemistry at the Royal Military School in Brussels in 1840. He had to retire in 1869 because of trouble with his voice through a throat ailment and became instead commissioner of the mint, but retired from this in 1872.

Stas was well known in his time for his extremely accurate determination of atomic weights. At first he supported William Prout's hypothesis that the weight of all elements is an exact multiple of that of the hydrogen atom. All his early measurements seemed to agree with this theory, but as his work progressed he seemed to be getting more and more fractional numbers and this turned him into the most articulate and damaging opponent of Prout. His work laid the foundations for the eventual formation of the periodic system.

Stas also carried out chemical analysis on potato blight and nicotine poisoning.