Hoel was born into the royal house of Brittany, the son of King Budic II, a ruler of the Armorican Britons whose kingdom covered the north-western peninsula of Gaul that the Britons had colonised in the late Roman period. His father was a man of sufficient power and piety to appear in several Breton saints' lives, and Hoel inherited both the throne and the military reputation that went with it. In Welsh tradition he is called Hywel, and the Welsh Triads name him as one of Arthur's three Royal Knights, a mark of the highest distinction in the early heroic tradition. He was said to be handsome, wise, and skilled at arms, and his court at Nantes was one of the great centres of Brittonic culture on the continent.

The defining moment of Hoel's military career came when Arthur, newly crowned and facing rebellion on every side, sent to Brittany for help. Hoel did not hesitate. He gathered fifteen thousand armed warriors, sailed to Southampton, and marched to join Arthur at the siege of Kaerluideoit, where the Saxons under Colgrim had the British forces pinned down. Together they broke the siege, and from that moment Hoel became Arthur's right hand in every campaign. He fought at the Battle of Dubglas, where the Saxon lines were shattered; at the Siege of Caer Ebrauc, the great fortress that the English call York; and at the Battle of Cat Celidon Coit, the battle of the Caledonian Wood. When Arthur was drawn away to fight elsewhere, Hoel was left to hold Dumbarton Castle against a combined force of Picts and Scots, and though he was sick and his garrison was outnumbered, he held the walls until Arthur returned to relieve him.

The conquest of Gaul was Hoel's greatest achievement. When Arthur turned his attention to the continent, determined to punish the Roman Emperor Lucius for demanding tribute, Hoel led one of the main divisions of the British army. He conquered Aquitaine and Gascony, subdued the Poitevins, and established Arthur's authority across the northern half of Gaul. At the battle of Saussy, the decisive engagement against the Romans, Hoel commanded the fourth division of Arthur's army, and his Breton warriors were among the fiercest fighters on the field. After the victory, Arthur made him governor of Gaul, a position of immense trust that reflected the bond between the two kinsmen. Hoel ruled the conquered territories with the same wisdom he had shown in Brittany, and for a time it seemed that the British empire on the continent might last.

The tragedy that broke Hoel's heart came not on the battlefield but on a mountain in Normandy. While Arthur and Hoel were campaigning in Gaul, news arrived that a giant of monstrous size had come from the shores of Spain, had carried off Helena, Hoel's niece, from her guard, and had fled with her to the top of what is now called Mont Saint-Michel. The giant had killed or devoured every soldier who pursued him, and Helena was already dead by the time Arthur heard the news. The king, taking only Sir Kay and Sir Bedivere, went out privately from the camp at the second hour of the night and climbed the mountain. He found the giant by a fire, his face besmeared with the clotted blood of swine, and after a terrible combat in which the giant nearly crushed him, Arthur struck his sword up through the monster's skull. Hoel, in grief for his niece, commanded a mausoleum to be built over her body on the mountain, and the place was called Helena's Tomb.

The end of Hoel's story is bound to the end of Arthur's. When Mordred seized the throne and declared himself king of Britain, Arthur was forced to abandon his continental conquests and return to deal with the traitor. He left Hoel in charge of Gaul with orders to keep the peace, and Hoel remained at his post even as the news came that Arthur had been mortally wounded at Camlann and borne away to the Isle of Avalon. In the later Prose Tristan tradition, Hoel's role expanded beyond the chronicles: he became the father of Isolde of the White Hands, the Breton princess whom Tristan married after he was banished from Cornwall, and of Kaherdin, the loyal companion who tried to bring Isolde of Ireland to Tristan's deathbed. In the Vulgate Lancelot, he is also called Aramont, the overlord of King Ban of Benoic and King Bors of Gannes, the two kings whose deaths at the hands of Claudas, the Frankish conqueror of Benoic and Gannes, set in motion the entire Lancelot saga. Whether as Arthur's cousin, his general, or the father of Tristan's second wife, Duke Hoel stands as the bridge between Britain and Brittany, the kinsman whose loyalty never wavered and whose sword conquered a continent.