A group of Northmen who were led by a ‘Prince of Rhovanion’ founded the Kingdom of Rhovanion before the year TA 490. Rhovanion at this time referred only to the region where he and his people dwelt, a strip of land east of Greenwood the Great and south of Dale and Esgaroth (Laketown). It is possible that Dale and Esgaroth formed part of the Kingdom before Erebor was founded by the dwarves of Durin's Folk.
The Men of Rhovanion fought with Gondor against the Easterlings of Rhûn, who threatened their lands, and when these were defeated by the Gondorian King Turambar in circa TA 550 Rhovanion became an ally of Gondor.
By TA 1200 the ruler of Rhovanion called himself the King of Rhovanion. In TA 1248 Rómendacil II of Gondor destroyed all camps of the Easterlings even beyond the Sea of Rhûn, and a strong alliance with Rhovanion was forged. Gondor withdrew its borders to the Anduin, and ceded all the lands east of Anduin to Rhovanion. From this time onward, the name Rhovanion referred to all of Wilderland.
The King of Rhovanion at this time was Vidugavia, and Prince Valacar of Gondor served in his army. Vidugavia's daughter Vidumavi married Valacar, and their son Vinitharya became King Eldacar of Gondor in TA 1432, which lead to the Kin-strife in TA 1437. Eldacar fled to Rhovanion, and with a Rhovanion army, he reclaimed his Kingdom in TA 1447.
In TA 1636 the Great Plague devastated Rhovanion, killing more than half its people. This left Rhovanion weakened, and in TA 1851 the Wainriders overran and enslaved Rhovanion. For 43 years Rhovanion was enslaved, but in TA 1899 Rhovanion revolted, while Gondor attacked the Wainriders from the west. Rhovanion was freed, but left extremely weakened. Many Men of Rhovanion left for Gondor, where they were welcomed as distant relatives.
Some of them may have gone north to Dale and Esgaroth after TA 1999, when Erebor was founded.
Around TA 2510 a new group of Easterlings, the Balchoth, appeared, and they quickly took the old lands of Rhovanion. This was the final end of the Kingdom of Rhovanion after over two thousand years of independence.