The Northmen who dwellt in Greenwood the Great and other parts of Rhovanion were friendly to the Dúnedain and were after the Dúnedain the most noble Men on Arda, counted as Middle Men by the Dúnedain, and believed to have been descended from the same group of Men as the Edain, the Atanatári. The only difference was that they didn't cross the Ered Luin into Beleriand and therefore didn't go to Númenor. The result of them not participating in the War against Morgoth was their considerably shorter lifespan if compared to the lifespan of the Dúnedain, whose lifespan was enhanced by the Valar after the War of Wrath.
For a time many of them even became subjects of Gondor, as the realm extended beyond the river Anduin.
East of Greenwood the Great was the kingdom of Rhovanion, and this became the most important nation of the Northmen. In the fourteenth century of the Third Age, King Rómendacil II of Gondor ensured their loyalty by crushing the Easterlings in Third Age 1248 and many Northmen joined the army of Gondor; he sent his son Valacar as an ambassador to Vidugavia, the so-called "King of Rhovanion". Valacar loved Rhovanion and the daughter of the King, Vidumavi. He married her. and she bore him a son whom she called Vinitharya in her mother tongue. In Gondor Vinitharya became known as King Eldacar and caused a civil war, the Kin-strife because of his mixed Dúnadan/Northmen blood in Third Age 1447.
Shortly after the death of Vidugavia a small group of Northmen moved to the vales of the Anduin between the Gladden Fields and the Mirkwood, probably to flee from the Wainriders, who had enslaved Rhovanion.
In Third Age 1856 Northmen were enslaved by the Easterlings but revolted in Third Age 1899, aided with a timely attack of King Calimehtar. Afterwards they battled with the enemies alone since Gondor could not help them. Still they joined the forces of Gondor and many comprised the army of the Battle of Fornost (Third Age 1975).
After the evil Kingdom of Angmar was defeated by Gondor and the remains of Arnor in 1977 T.A., these Northmen moved north and began to call themselves the Éothéod. They were skilled horsebreeders and horsemen. In 2510 T.A. they responded to a plea of help from the trapped Gondorian army at the Field of Celebrant. After they helped Gondor win this important victory they were rewarded the province of Calenardhon and became known as the Rohirrim.
The later centuries it is mentioned that the Northmen were allied with Erebor and with Dwarven weapons drove the enemies back to Rhûn. However they did not prevent the enemies to reach the gates of Erebor during the War of the Ring.