Adminius was a son of Cunobelinus, ruler of the Catuvellauni, a tribe of Iron Age Britain. His name can be interpreted as Celtic *ad-mindios, "to be crowned".

In the early to mid 1st century AD Adminius was ruler of eastern Kent, a kingdom which presumably fell within his father's sphere of influence, but was deposed around AD 39 or 40. The elderly Cunobelinus, who had maintained friendly relations with the Roman Empire, may have lost control to an anti-Roman faction led by his his other sons, Togodumnus and Caratacus, who may have been instrumental in forcing Adminius out of power. Alternatively, his fall may have been the result of a revolt of the Cantiaci against Catuvellaunian rule. Adminius fled to continental Europe with a small group of followers and surrendered to the Romans. The emperor at the time, Caligula, presented this relatively minor event as a great victory over the foreign tribes of Britain and even penned an extravagant report which he insisted was read to the Roman senate.

Adminius appears to have persuaded Caligula that Britain was vulnerable to attack and that an invasion would be an even more famous victory for him. It is likely that the capture of the British prince was the germ of Caligula's initiative to launch an invasion of Britain. The invasion never happened either because of Caligula's famous eccentricity which Roman historians record led him to order his army to collect seashells from Gaulish beaches as war trophies, or because of a mutiny in the invasion force assembled at Boulogne.

In any case, Rome's refusal to return the fugitive Adminius to his father was one of the contributary factors to growing anti-Roman sentiment in Britain which necessitated Claudius' successful invasion in AD 43.