Geoffrey of Monmouth introduces Belinus after the death of Dunvalo Molmutius, the lawgiving king whom Geoffrey calls Dunwallo Molmutius. Belinus and his younger brother Brennius both seek their father's throne. Their supporters first divide the kingdom, giving Belinus the crown and the greater share because he is the elder, while Brennius rules the north under him. The arrangement collapses when Brennius acts independently and the brothers go to war. Geoffrey's narrative is pseudo-history written in the twelfth century, so its battles and chronology are part of a legendary account of Britain's past rather than evidence for a fourth-century BCE monarchy.

The brothers' conflict reaches its decisive stage after Brennius returns from Gaul with a new army. Geoffrey tells how their mother, Conwenna, intervenes before another battle and pleads with Brennius to forgive his brother. The appeal succeeds: Brennius embraces Belinus and the civil war ends. The reconciliation becomes the turning point of the story because the two kings then direct their strength outward. Together they cross to the continent and campaign through Gaul, presenting a medieval vision of British power projected backwards into antiquity.

Geoffrey next carries Belinus and Brennius into conflict with Rome. Roman consuls named Gabius and Porsenna agree to tribute and hostages, but the Romans later break the settlement while the brothers are campaigning elsewhere. Belinus intercepts the Roman force, and the brothers eventually reach Rome and besiege the city. In Geoffrey's account they take the city after hard fighting. This episode is one of the most dramatic inventions of Britain's legendary chronicle tradition; it should not be treated as a record of an otherwise unknown British conquest of republican Rome.

Belinus then returns to Britain and is portrayed as a builder and lawgiver. Geoffrey credits him with founding or rebuilding Kaerusk on the River Usk, identified in the Welsh tradition with Caerleon-on-Usk, and with constructing a remarkable gate and tower beside the Thames at Trinovantum. The chronicler says the citizens called the gate Belinesgata after the king, later understood as Billingsgate. Modern scholarship does not accept that legendary etymology as the historical origin of the London place-name, but the association became part of Belinus's literary identity. Geoffrey also says he reaffirmed his father's Molmutine laws and that his ashes were placed in a golden urn on the tower.

Welsh adaptations preserved the story but adjusted the name. Brut y Brenhinedd renders Geoffrey's Belinus as Beli ap Dyfnwal Moelmud, while Welsh scholarship also records the forms Belin and Belyn as more appropriate equivalents. This Belinus should not be merged with Beli Mawr, the legendary ancestor connected with Lludd and Caswallon, whom the Welsh Bruts use in place of Geoffrey's later king Heli. Keeping those figures separate preserves two different strands of British legendary genealogy: Belinus, son of Dunvalo Molmutius and brother of Brennius, and Beli Mawr, the ancestor attached to a later part of Geoffrey's king-list.