The name Demario has its origins in the Romance languages, derived from the Latin name Demetrius, itself a borrowing from the Greek name Dēmētrios. The name Dēmētrios is believed to have been formed from the combination of the Greek words dēmos (meaning "people") and the name of the goddess Demeter, the ancient Greek deity of agriculture and the annual cycle of crops and fertility.

Demario is a variant spelling that emerged in Italy during the Middle Ages, likely through the influence of Italian and other Romance dialects on the original Greek and Latin forms. It was a popular name among the nobility and aristocracy in regions like Tuscany and Umbria, but also found use among commoners across the Italian peninsula.

One of the earliest recorded instances of the name Demario can be found in the writings of the 13th-century Italian poet and philosopher Dante Alighieri, who mentions a character named Demario in his epic poem the Divine Comedy. In the 14th century, a Demario di Guido was a renowned painter and fresco artist active in the city of Siena.

During the Renaissance period, the name gained further prominence with figures like Demario Beccadelli (1486-1549), an Italian philosopher and humanist scholar, and Demario Cavalcanti (1538-1607), a renowned Italian mathematician and astronomer who made important contributions to the study of optics and mechanics.

In the 17th century, Demario Genovesi (1642-1719) was an Italian jurist and legal scholar who served as a judge in the Grand Duchy of Tuscany, while Demario Grimaldi (1688-1757) was a notable Italian architect and engineer who designed several churches and palaces in his native Genoa.

As the name spread beyond Italy, other notable figures emerged, such as the French painter Demario Delacroix (1798-1863), whose works were pivotal in the development of Romanticism in art, and the Spanish composer Demario Granados (1867-1916), known for his piano compositions and zarzuelas (Spanish operettas).