Before the Roman invasion of the area around AD43, a confederation of Celtic tribes known as the Brigantes ruled Britain from the Humber to the Firth of Forth. According to Ptolemy, their territory spanned the island.

Tacitus recorded that When the Romans first arrived, they encountered a tribal federation under Cartimandua, sovereign queen of Brigantia, and Venutius, her warlord husband. Cartimandua agreed to an alliance with the Romans and her land prospered as a client state. In 51 AD Caratacus, leader of southern resistence to Roman rule, was captured and Cartimandua immediately turned him over to the Romans. This made Caratacus somewhat of a martyr among the anti-Roman forces while Tacitus praises Cartimandua as "having secured the most important component of Emperor Claudius' triumph." When Cartimandua divorced and remarried, Venutius, formerly loyal to the Romans, turned against them and gathered enough support among the Brigantes to cause a civil war among the federation. He almost won, but the Romans came to Cartimandua's aid at the last minute. However, she was forced into exile and around the year 69AD Venutius became king of Brigantia. His reign was short-lived because the Roman army finally vanquished him a few years later.

In AD71 the Romans set up a fortress at the confluence of the Rivers Ouse and Foss. This fort, known as Eboracum, held 6000 soldiers within its 50 acres. Around the fort grew a town that would become one of the leading cities of the Roman Empire. The Brigantian territory became the Roman province of Britannia Secunda.

After the Romans left Britain, Britannia Secunda reverted to tribal territories again, divided among the Parisi, Carvetii, and Brigantes. The dynasty of Coel Hen the Protector united the region and extended it beyond the boundaries of Roman rule to encompass Gododdin and some of Galloway. Through the fourth and fifth centuries, abandoned Roman forts were reclaimed by local tribes and sometimes guarded by Saxon mercenaries. By the Age of Arthur (5th-6th centuries) the Brigantians had built up strength, forming a kind of federation of alliances that once again made them a major power.