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.
The Northmen descended from the group of the Edain who left their kin to reach the West. However these people decided to stay behind. These were divided in two folks (who would later become House of Bëor and House of Marach) at the shores of the Sea of Rhûn. The Greater Folk dwelt in the north-east of the Sea, in the woods that there came near to the shores and the Lesser had reached the feet of the high hills to the south-west. Because of that distance, the Greater House developed a craft of boat-building.
When they continued their trail, many sub-tribes of both peoples stayed behind, and when the shrinking teams (now led by Bëor and Marach) reached Beleriand, those who stayed behind occupied many parts of northern Rhovanion. These appear to be akin to the House of Hador.
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.
Their existence was a bulwark of Gondor, keeping its northern and eastern frontiers from invasion. When they were weakened and at last destroyed, first with the Great Plague, which appeared there in the winter of III 1635 and soon spread to Gondor. The Horsemen suffered since the Plague came with a cold winter and they had only low wooden houses and thronged stables, little skilled in the arts of healing and medicine. When the Plague passed more than half of the foil of Rhovanion had perished, and of their horses also. They were slow to recover undisturbed, since the people further east had been equally afflicted.
When the invasions of the Wainriders began against Gondor, the Northmen suffered the first assaults. Their scattered remains were recruited by King Narmacil II who took a great army north into the plains south of Mirkwood. They were defeated in the Battle of the Plains where Narmacil and Marhari were killed. The remnant of his army retreated over the Dagorlad into Ithilien. A few Northmen fled to Gondor, others over the Celduin and were merged with their kin, the folk of Dale under Erebor and others followed Marhwini to the Vales of Anduin and became the ancestors of the Éothéod.
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 III 1977, these Northmen moved north and began to call themselves the Éothéod. They were skilled horsebreeders and horsemen. In III 2510 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.