He was originally armour-bearer to Venutius, husband of Cartimandua, the queen of the Brigantes and an ally of Rome. Some time after 51 AD Cartimandua split with Venutius, marrying Vellocatus and elevating him to kingship. Venutius went on to become an important figure in the resistance to Roman occupation.
Venutius staged two revolts against Cartimandua, first in the mid-50s, which was defeated by the Romans, and again in 69, this time successfully. Cartimandua was rescued by the Romans as Venutius seized power. Vellocatus's fate is not recorded.