Brutus first appears in the Historia Brittonum, composed by the Welsh monk Nennius around 829 CE, where he is named as the son of Silvius, grandson of Ascanius, and great-grandson of Aeneas, the Trojan hero who settled in Italy after the fall of Troy. The full biography that made him famous was written three centuries later by Geoffrey of Monmouth in his Historia Regum Britanniae, composed around 1136. Geoffrey claimed to be translating an ancient book, and whether he was or not, his version became the definitive account of Britain's mythical origins.

In Geoffrey's telling, Brutus was born in Alba Longa in Italy, and before his birth a sorcerer prophesied that he would be a man of great renown who would also be the death of both his parents. His mother died in childbirth, and the prophecy was fulfilled when Brutus, aged fifteen, went hunting with his father Silvius and accidentally shot him with an arrow. For this crime he was banished from Italy, and he wandered through the Mediterranean seeking a new home for the Trojan blood that flowed in his veins.

He came to Greece and found a community of Trojan descendants who had been enslaved by King Pandrasus since the fall of their city. Brutus rallied the enslaved Trojans, attacked Pandrasus's camp by night, overwhelmed the guards, and captured the king. Pandrasus, desperate to save his life and rid himself of these rebels, agreed to set the Trojans free, gave them ships and supplies, and offered his daughter Ignoge in marriage to Brutus as a pledge of peace. The Trojans crowned Brutus as their king, and with Ignoge at his side he set sail into the western ocean.

The voyage brought them to a deserted island called Leogecia, where Brutus found a ruined temple to the goddess Diana. He performed the sacred rites and fell asleep before her statue. Diana appeared to him in a dream and showed him a rich and fertile island in the western sea, inhabited by only a few giants, where he and his descendants would found a kingdom that would spread across the world. She named the island Albion, and promised that Brutus would be the first of a long line of kings. He woke with the vision burning in his mind and ordered his fleet to sail west.

They landed on the coast of what is now Devon, where Brutus stepped ashore onto a stone and declared: Here I stand and here I rest, and this town shall be called Totnes. The Brutus Stone still lies in the pavement of Fore Street. The island was everything Diana had promised: green, fertile, and ruled by a dwindling race of giants. Brutus and his Trojans fought the giants and drove them back with disciplined tactics and bronze weapons. At a festival of thanksgiving the giants launched a surprise attack, but Brutus rallied his men and slaughtered them all except their leader Gogmagog, a giant twelve cubits tall who could uproot an oak tree. Brutus spared Gogmagog so that his lieutenant Corineus, a man of superhuman strength who delighted in fighting giants, could face him in single combat. The two grappled on a cliff top, Gogmagog breaking three of Corineus's ribs in a mighty bear hug, but Corineus lifted the giant across his shoulders and hurled him onto the rocks below, where the sea turned red with his blood.

With Albion free of giants, Brutus named the island Britain after himself, and his people became the Britons. He travelled up the River Thames and built Troia Nova, New Troy, a city that would later become London. He divided the island among his three sons: Locrinus received the centre called Loegria, Kamber received the west called Cambria, and Albanactus received the north called Albany. He gave laws to his people, established a kingdom of justice and peace, and ruled for twenty-four years. His descendants, Geoffrey wrote, would include King Lear and in time the once and future King Arthur himself.