Barinthus first appears in Geoffrey of Monmouth's Vita Merlini, a Latin poem composed around 1150. Taliesin describes to Merlin what happened after the battle of Camlann, where Arthur was mortally wounded by his son Mordred. Taliesin says that he and others took the wounded king aboard a ship, and that Barinthus was the steersman because of his knowledge of the seas and the stars of heaven. With Barinthus at the tiller, they crossed the western ocean and arrived at the Isle of Avalon, where Morgen, the eldest of nine sisters, received them. She laid Arthur on a golden bed, examined his wound, and declared that he could be healed if he stayed with her. Taliesin and the others then spread their sails to favourable winds and returned, leaving Arthur in Morgen's care and Barinthus's name linked with the king's last voyage.

The name Barinthus carries echoes of an older Irish tradition. A figure named Barrintus appears in the Navigatio Sancti Brendani, the Voyage of Saint Brendan, a seventh-century Irish text about the saint's journey to a blessed island in the western ocean. In that tale, Barrintus is a holy man who had already visited the island and who describes its wonders to Brendan, inspiring the saint to make his own voyage. Geoffrey of Monmouth may have drawn on this tradition, transforming the Irish pilgrim into the Arthurian pilot who knows the way to the Otherworld. The connection suggests that Barinthus rested on a genuine Celtic belief in navigators who could cross the boundary between the mortal world and the land of the blessed.

In the Vita Merlini, Barinthus is described only in a single line, yet that line is enough to make him significant in the Arthurian afterlife. He is the man who knows the seas and the stars, the pilot whose skill is the difference between a lost king and a saved one. The poem does not tell us where he came from, how he learned his craft, or what became of him after the barge turned for home. He appears, performs his function, and vanishes from the narrative, a figure as silent and essential as the tiller itself.

The image of Barinthus at the tiller has echoed through centuries of Arthurian retelling. In later romances, the journey to Avalon becomes more elaborate, with three queens weeping and the last knight standing alone on the shore, but the pilot remains. Tennyson, in the Idylls of the King, gave the scene its most beautiful modern form, though he did not name Barinthus, preferring to let the barge itself carry the emotional weight. In T.H. White's The Once and Future King, the journey is described with a matter-of-factness that makes it more moving, the wounded king carried away by the simple competence of men who know their business. Barinthus is the archetype of that competence, the professional navigator who does not need miracles because he has the stars.

Barinthus's legacy is the journey itself, the proof that Arthur's death was not an ending but a passage. Every retelling of the Avalon legend depends on the assumption that someone knew the way, that the western ocean led to a place where wounded kings could be healed. Barinthus is that someone, the man who made the passage possible. Without him, Arthur would have died on the battlefield. With him, the king became the once and future king, the sleeper who waits beneath the hill, ready to return when Britain calls. The pilot who steered by the stars gave Arthur his immortality.