Queen Cartimandua is first mentioned by Tacitus in CE 51, but her rule over the Brigantes may have already been established in 43 when Claudius began the organized conquest of Britain: she may have been one of the eleven "kings" who Claudius' triumphal arch says surrendered without a fight.

If not, she may have come to power after a revolt of a faction of the Brigantes was defeated in 48 by Ostorius.

Of "illustrious birth" according to Tacitus, she has probably inherited her power as she appears to have ruled by right rather than through marriage.

She and her husband, Venutius, are described by Tacitus as loyal to Rome and "defended by our [Roman] arms".

Caractacus, his forces defeated by Ostorius in North Wales, flees to Cartimanduas, but in 51 she turns him over, in chains, to the Romans, who have supported her in a number of anti-Roman revolts among her subjects.

Caratacus is sent to Rome as a war prize, presumably to be killed after a triumphal parade.