Thallium was discovered in 1861 by Sir William Crookes, through spectroscopic techniques, during his seek for tellurium, in some plant residues. Crookes found a sharp and unknown green stripe in the spectrum of some acids removed from those plants, and proposed the existence of a new element. This green stripe reminded Crookes, the color of the vegetation in Spring. For that reason, he called it thallium, that derives from the Latin word thallus, which means "in bloom". During that year, Crookes isolated a small amount of metallic thallium, and exhibited it at an international conference in London, in 1862.

The French professor Lamy was also well successful in identifying this new element by the spectroscopic techniques of Crookes.

At first, thallium was thought to be an element from the sulfur family. However, this erroneous idea was quickly abandoned since it possessed properties similar to those of lead, mercury and aluminum, its correct position being attributed in the periodic table in group IIIA.