During the Third Age, first mention of the Éothéod is when remnants of the Northmen, after the Battle of the Plains followed Marhwini to the Vales of Anduin, soon joined by many fugitives from Mirkwood whose lands were taken by the Wainriders. Gondor didn't know about their existence for many years.
Marhwini warned Calimehtar that the Wainriders were plotting to raid Calenardhon over the Undeeps but the enslaved Northmen also prepared a revolt against the Wainriders. Calimehtar therefore provoked the Wainriders out of Ithilien, and his horsemen, joined by a alrge éored led by Marhwini, drove the Wainriders back.
Desperate poorly-armed outlaws came out of Mirkwood and roused the slaves. They burned many Wainriders dwellings, storehouses and fortified camps of wagons. Most of them perished in the attempt fighting the Wainriders' youths, women and old men. Marhwini retired to his land beside the Anduin, and the Northmen of his race never again returned to their former homes.
Forthwini was troubled by raids into the south of his land, both up the the river and through the Narrows of Mirkwood and warned King Ondoher that the Wainriders were recovering. The attack finally came and men of the Éothéod fought with Ondoher. Faramir son of Ondoher refused to remain in Minas Tirith and joined a battalion of Éothéod but was caught with a party of them as they retreated towards the Dead Marshes. The leader of the Éothéod did not manage to save him and Faramir died in his arms, before he found out that he was the Prince. He then went to join Minohtar at the head of the North Road in Ithilien. Eventually the Wainriders were defeated by Earnil II at the Battle of the Camp.
The Éothéod were now well-known to Gondor as a people of good trust. They informed Gondor of all that passed in that region. After the fall of Angmar they moved under Frumgar to a new land north of Mirkwood, between the Misty Mountains and the Forest River away from the ravages of the Easterlings and Orcs. Southward it extended to the confluence of Greylin and Langwell sources of the Great River Anduin, near where the Ered Mithrin met the Misty Mountains. They founded their capitol there.
Some time later their king Fram, son of Frumgar, slew the dragon Scatha. The Éothéod capital was named Framsburg in his honour. After Fram nothing is known of the leaders of Éothéod until much later time, when king Léod was killed trying to tame the horse Felaróf, first of the Mearas of Rohan. His son and successor Eorl the Young tamed the horse, taking it into service as compensation for his father's life.
During the rule of the ruling Steward of Gondor Cirion had always in his mind the menace from the North. It was thus not until the winter of III 2509 was past that he became aware that a great movement against Gondor was being prepared. Gondor faced an attack by the evil Balchoth, and Cirion sent messengers to the Éothéod capital. King Eorl answered the call for help, and rode out with most of the Éothéod to help their allies of old, leaving only a few warriors behind to protect his people. The Riders arrived just in time to help the army of Gondor at the Field of Celebrant, and after defeating the enemy Cirion asked the Éothéod to watch over the depopulated province of Calenardhon.
Three months later Cirion gave Calenardhon as a gift to Eorl and his people, and Eorl swore his Oath of eternal friendship. Messengers were sent north, and the Éothéod completely removed to the plains of Calenardhon.