Myonia is attested by classical sources as a polis, thus it must have been a political community,[2] one of the principal of the ancient Greek tribe of Locrians, in the region of Ozolian Locris.

In 338 BC, Myonia was sacked by Philip II of Macedon along with Amfissa, because Ozolian Locrians had illegally cultivated part of the Crissaean plain which belonged to Delphi.

The Amfissians, who were alarmed at the hostility of the Phocians, gave hostages to him and induced the other Locrian cities to do the same; the first of them were their neighbours, the Myonians, who held the most difficult of the passes.

The town was lying upon a hill, and it had a grove and an altar sacred to the gods called Meilichioi, the sacrifices to whom were offered at night and the rule was to consume the meat on the spot before sunrise.

He concludes that the Myanians were the same folk as the Myonians of the Locrian mainland, as he remembered the reference to the latter by Thucydides, and says that the letters on the shield were a little distorted, a fault due to the antiquity of the votive offering.

Louis Robert, in his work Études épigraphiques et philologiques, places Myonia at Agia Efthymia,[8] as well as The Princeton Encyclopedia of Classical Sites,[9] Alfred Philippson, and the editors of Inscriptiones Graecae, Günther Klaffenbach and Johannes Kirchner.

Other revolutionaries from Agia Efthymia who are popular in the local folk tradition are Arapogiorgos, the guerilla chief during 1750–1760, Mitros Dedousis around 1770, Georgios and Giannis Karaplis by the end of the 18th century.

The ancient name of the village was Myonia (Μυωνία, Μυονία), also referred to as Myania (Μυανία), and it was held until 1580, slightly altered to Mynia (Μυνιά).

In 1928, at Kazas, two iron swords within two graves were discovered, as well as lances and javelins and a bronze helmet of the 6th century BC, located in the Archaeological Museum of Amphissa.

The village is situated on the foothill of Mount Giona, the highest mountain of Southern Greece, on the site that the ancient town existed.

Agia Efthymia is also close to some popular destinations of Greek and foreign travellers: the famous archeological site of Delphi, the coastal towns Itea and Galaxidi, Parnassos Ski Center.

Now the population of the village has reduced to 374 residents according to the census of 2021, although the people originating from it who visit and live there during the year, especially in summer, increase the number to more than 1,000.

Giannis Skarimpas, a popular writer, was born and raised in Agia Efthymia, descended from a well-known local family which had taken part in the Greek War of Independence.

Ioanna Glymi, a professional painter who had taken part in various contests in Paris, also descended from Agia Efthymia and left to the community approximately 100 of her paintings.

Other distinguished people who descended from the village were the hero of the Greek War of Independence Astrapogiannos, the painter Georgios Kalamaras and - as it is said -the popular folkloric personality Maria Pentagiotissa.