In the classical period, it was first occupied by the [Gaul|Gallic]] people, the Andecavi, before Caesar conquered it. In the 5th century it fell to the Franks, and it became a county under Charlemagne.
Lambert of Nantes sought to carve out a principality for himself in the mid-ninth century; after his death the duke of Brittany, Erispoe, took the territory over, and it was handed down in his line. At the same time the dukes of Normandy also coveted the lands and attacked incessantly. Toward the end of the century one Fulk the Red was made viscount and, he holding on to the lands (although he lost the county of Nantes), he assumed the title of "count". His son (another Fulk) and grandson (Geoffrey) succeeded him and the latter successfully made Nantes his vassal and obtained the district of Loudon in fief.
Fulk III (Geoffrey's son) threw off attacks by the counts of Blois and of Rennes, as well as holding off the French king, Robert the Pious. Fulk's son, Geoffrey Martel, continued the expansion: in 1051 he beat the counship of Maine into submission, and obtained recognition of his authority, although he was not, at that time, able to obtain revenge on Normandy (then under Duke William, the Bastard.
Fulk's line contined to hold Anjou until 1113, when, after a battle at Alencon, Fulk V was obliged to recognise Henry I of England as his liege lord. Henry later married his son, William, to Fulk's daughter, Matilda. After William's death, and some further complexities, a second marriage, between Henry's daughter Matilda, widow of the Holy Roman Emperor Henry V, and Fulk's son, Geoffrey. Then, on the invitation of Baldwin II, king of Jerusalem, Fulk set sail for the Holy Land, where he was to marry Baldwin's daughter, Melisende. Geoffrey, meanwhile, adopted the surname "Plantagenet", made substantial inroads into the territories surrounding his own, until only the French king was his overlord (and that more by sufferance than obligation), and founded a pocket empire for himself, becoming a duke and taking over Normandy in 1144. His son, Henry Curtmantle, was gifted the duchy in 1149 (his father died in 1151) and in 1154 he ascended the English throne as Henry II.
On the death of Henry's son Richard I, Arthur of Brittany claimed Anjou, over the succession of John Lackland of England. A war ensued: John was recognised count by Philip Augustus in 1200, only to lose it to Arthur when John refused to do homage to Philip in 1202. Arthur's death meant the duchy came under the French crown. In 1246 Louis IX of France gave it, as an appenage, to his nephew Charles, then heading for the thrones of Naples and Sicily.
Later Charles of Valois obtained the duchy, among the dowry of his wife, Margaret, daughter of the king of Naples; he left it to his son, Philip, who reunited it with the French crown when he became king of France in 1328. Thereafter the title and territory remained within the extended French royal House, being granted out and taken back.