The Myonians were mentioned by Thucydides in his work History of the Peloponnesian War (History of the Peloponnesian War/Book 3, par.101). When the Spartan general Eurylochus, on the road to Naupactus, arrived to Delphi, he sent a herald to Amphissa, because the road passed through the land of the Ozolian Locrians and he also wanted to detach them from Athens. The Amphissians were alarmed at the hostility of the Phocians and they first gave him hostages themselves and persuaded the other Locrian cities to do the same; the first were their neighbours, the Myonians, who held the most difficult of the passes.
Pausanias, in his work Description of Greece (Phocis and Ozolian Locri, XXXVIII. 8), referred to Myonia as a town farther inland from Amphissa and above it, thirty stades away. The town was lying upon a hill and it had a grove and an altar of the "Gracious Gods", the sacrifices to whom were offered at night and the rule was to consume the meat on the spot before sunrise. Beyond the city, there was a precinct of Poseidon, called Poseidonium, with a temple of the god in it, the image of which had disappeared before the author's time. In another book of the same work (Elis B, XIX. 4, 5), Pausanias stated that there was a bronze-plated shield in the Temple of Zeus at Olympia, adorned with paintings on the inner side and along with the shield were a helmet and greaves. An inscription on the armour said that they were "dedicated by the Myanians as first fruits to Zeus". He concluded that the Myanians were the same folk as the Myonians of the Locrian mainland, as he remembered the reference to them by Thucydides, and said that the letters on the shield were a little distorted, a fault due to the antiquity of the votive offering.
There had been some confusion during the 19th and early 20th century about what the original location of ancient Myonia is. Several historians and specialists on the geography of ancient Greece have identified Myonia as the village of Agia Efthymia, so there is no doubt about it anymore. Louis Robert, in his work Études épigraphiques et philologiques, in 1938, located Myonia at Agia Efthymia. Other scholars who have supported the same are Günther Klaffenbach and Johannes Kirchner, editors of Inscriptiones Graecae, and Alfred Philippson.
The disobedient character and resistance of the local people against the Ottoman Empire were strong and admirable. Agia Efthymia was a center of armatoloi. Around 1705 the leader was Katsonis, uncle of the hero Lambros Katsonis, who was nicknamed "Vrykolakas" meaning 'vampire', because until his death in 1740, he was so much feared by the Ottoman Turks that they left the whole territory to his jurisdiction in order to free themselves from him. The famous Greek writer Andreas Karkavitsas had dedicated to Vrykolakas a whole story called The Exarch.
By the end of the 17th century obe of the total eleven attempts of independence took place in the region of Agia Efthymia, headed byt the Bishop of Salona, Philotheos Charitopoulos, who was born in the village. He, together with captain Kourmas, formed an alliance with Venetian army and navy, trying to overthrow the Ottoman domination when he was killed in the battlefield.
The man who took charge after Charitopoulos' death was his son-in-law, Kostas Zacharias, nicknamed "Konstantaras". His achievements were sung in popular folk songs. The poet Kostas Krystallis wrote the story Captain Konstantaras, in which he referred to the fact that Konstantaras killed his only left son - his other two sons had been killed by the Ottomans - with his own hands because he dishonored his family while he had been living on the island of Agios Konstantinos, opposite of Itea, where he had been sent to attend the local school. Konstantaras died in 1755 and his jurisdiction was shared to his three - second in command - men.
The tradition of the local fighters was continued by the famous fighter Astrapogiannos, who was also born in Agia Efthymia and his real name was Giannos Zacharis. Astrapogiannos was praised by popular folk songs too and the poet Aristotelis Valaoritis was inspired by Astrapogiannos' last moments after he was wounded. He wrote the poem Astrapogiannos in 1867, in which he praises the friendship between Astrapogiannos and his lieutenant, Lambetis. There was also a popular Greek film produced in 1970, titled O Astrapogiannos, dedicated to the fighter.
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. During the Greek War of Independence, several fighters from the village were distinguished, taking part in some of the most important battles like the battle of Gravia, the battle of Vassilika and the battle of Alamana. Among them Ioannis Kalpouzos and his son Anagnostis Kalpouzos, with the latter being killed during the battle in the region of Alamana, where he fought along with Athanasios Diakos, which made the first king of Greece Otto honor Kalpouzos family by visiting them in Agia Efthymia.
During World War II, Agia Efthymia suffered heavily from the Axis Powers as thirty-seven people were directly shot to death by the conquerors while the total loss of people numbered 120 individuals. On April 25, 1941, the village was bombed and its cathedral, dedicated to the "Birth of Theotokos", was damaged. On April 9, 1943, the troops of Fascist Italy burned the largest part of the village, including the church of Agios Efthymios, and in August 1944 the troops of Nazi Germany burned the rest of it. According to official data 365 out of the total 423 houses of the village were totally burnt and 20 partially. The State recognized the sacrifice of the Agioefthymiotes and in 2000 Agia Efthymia was included in the list of "Towns and Villages of Martyrdom" and a monument for the people who died in World War II was erected at the entrance of the village.
According to the "Galaxidi Chronicle", compiled by a monk named Efthymios in 1702, a huge earthquake in 1580 destroyed several towns in Phocis like Salona (Amfissa), Galaxidi, Lidoriki and Mynia. The desperate residents of the latter left the village and went to the mountain above it crying. There an old man with a long beard appeared and told them to return to their village and not to fear because he would protect them. This old man was believed to be Agios (Saint) Efthymios and actually he is always portrayed with a long beard. The residents returned to the village, rebuilt their houses and gave the name of the saint to their community, Agia Efthymia. Additionally they built a church dedicated to him.
It is interesting that Agia Efthymia is feminine and not the name of the old man. Besides there is no female saint in christianity with this name. The prevalent explanation of why the village was named Agia Efthymia instead of Agios Efthymios, is that the inhabitants might attempt to match the name of their village Mynia, which is feminine, with the feminine name Efthymia. Indeed, nowadays this seems very sensible since the residents call their village Aithymnia which sounds very close to the ancient name Mynia.
The residents engages mainly in agriculture, with products like the popular olives of the region, pastoralism, building, while many of them work in the bauxite mines. During the 19th and 20th century, Agia Efthymia was one of the largest villages in Parnassida and the whole of Phocis. Now the population of the village has reduced to 597 residents according to the census of 2001, 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. The drift of its population from 1821 to 2001:
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. Each year the society of the village organizes a series of cultural events called "Skarimpeia", dedicated to the writer. 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.