One of only two British women to be mentioned in the ancient sources, namely the Annals of Tacitus, the other being Boudicca, mentioned by Dio. She was the leading noble of the Brigantian federation who was probably granted a clientship with Rome in 43AD, along with her consort Venutius of the Carvetii. During the spring of 48AD, political pressure from certain rebels within the Brigantian nobility forced Ostorius Scapula to abandon his campaign against the Ordovices in north Wales, and turn his attention to the Brigantes (Tacitus Annals XII, 32). In 51AD Cartimandua tricked and captured the Catuvellaunian warlord Caratacus, then honoured her agreement with Rome by surrendering him to Scapula, who was still governor (Tacitus Annals XII, 36). This action seems to have stirred up some resentment towards the rule of Cartimandua within the Brigantian nobility, which for a period, lurked beneath the surface of the seemingly calm tribal pool.
In circa 53AD, however, Venutius divorced Cartimandua and formed a faction of his own amongst his Carvetian tribespeople, attacking Cartimandua's power-base and causing the new Roman governor, Aulus Didius Gallus to send a number of auxiliary cohorts to her aid (Tacitus Annals XII, 40). Cartimandua continued to rule the Brigantes for a number of years with her own armour-bearer, Vellocatus, as her consort.
However, the seeds of discontent were still germinating in the ranks of the Brigantian ruling houses for another rebellion occurred during the governorship of Marcus Vettius Bolanus around 70AD. During this uprising, Cartimandua had to be rescued by an ala of Roman auxiliary cavalry sent specifically for this purpose by the governor who was occupied against the Silures in south Wales. This continuing Brigantian unrest caused the emperor Vespasian to annul the clientship of the Brigantes and for the first time, they came under the direct rule of Rome.