The Thames was sacred long before the Romans arrived. The Celtic Britons called it Tamesa, from a Brythonic root meaning "dark" or "smooth," and they treated it as a living thing with indwelling spirits who must be acknowledged and nurtured. Archaeologists have recovered thousands of votive objects from its waters: polished stone axe heads, bronze swords, iron spearheads, and precious metalwork cast into the river as offerings to the powers beneath. The Thames was a transitional space, a door between the world of the living and the Otherworld of the dead, and the Celts buried some of their dead in its waters and deposited treasures at its confluences to secure the goodwill of the river spirits. The river was never merely geography; it was a member of the tribe, a deity in liquid form whose moods determined the fortune of every settlement along its banks.

When the Romans came, they recognised the sacred nature of the Thames and gave it a deity worthy of its stature. They Latinised the name as Tamesis, and the river acquired a tutelary god: Old Father Thames, a figure with flowing beard and hair who resembled the river gods of the Nile and the Tiber, carrying an urn that symbolised the plenty and renewal that the river brought to the land. The Romans also associated the Thames with other divine powers. An altar dedicated to Jupiter Optimus Maximus was found on the banks at Little Wittenham, and a ritual carving of a goddess was excavated at Bablock Hythe. The confluences of the Thames with its tributaries were especially holy ground, guarded by the three seated goddesses known as the Matres and by Condatis, the Celtic god of the watersmeet whose name means literally "god of the two streams." At the mouth of the Kennet, where the tributary joins the main river at Reading, offerings were made for centuries, and the Horseshoe Bridge that spans the confluence still carries an ancient symbol of fortune. The Thames had no single Celtic goddess of its own, unlike the Severn which was guarded by Sabrina or the Dee by Deva, but the absence only made the river more mysterious, a deity whose true name was hidden.

The Thames entered the legendary history of Britain through Geoffrey of Monmouth's Historia Regum Britanniae, written around 1136. In Geoffrey's account, Brutus, the grandson of the Trojan hero Aeneas, sailed to Britain after the fall of Troy and came to the Thames, where he founded a city called Troia Nova, New Troy, which would later be known as London. The river was thus woven into the very foundation myth of Britain, the water that received the first Trojan settlers and the boundary that defined the capital of the realm. The sixth-century chronicler Gildas, writing centuries before Geoffrey, had already called the Thames the guardian river, the river of boundaries, the place where kings met to negotiate and where religious synods were held. The same liminal quality that made the Thames sacred to the Celts made it politically essential to the Saxons and the Normans who followed.

The Christian transformation of the Thames began in the fourth century with the martyrdom of St. Alban. The Venerable Bede and the earlier chronicler Gildas recorded that Alban was led to execution on 20 June 304, but the bridge across the Thames was so crowded with spectators that he could not cross. Alban prayed, and the river parted before him, its waters standing upright like walls on either side, creating a dry path across the channel. The miracle echoed the parting of the Red Sea and the crossing of the River Jordan, and it placed the Thames firmly within Christian sacred geography. In the seventh century, St. Birinus converted the first Saxons to Christianity by baptising them in the Thames at places such as Somerford Keynes, Taplow, and Dorchester, establishing the river as a fountain of grace. Yet the old spirits never wholly departed. As late as the eighteenth century, statues of Old Father Thames adorned bridges and public buildings, and royal pageants on the river featured nymphs and sea gods in costumes that preserved the memory of the water deities. T.S. Eliot would lament in The Waste Land that "the nymphs are departed," but the lament itself proved that the memory of the river's sacred inhabitants still haunted the English imagination.

The source of the Thames is a place of ancient enchantment. At Thames Head in the Cotswolds, a small group of stones marks the origin, and an ash tree has stood there for approximately two centuries guarding the spot. In Norse mythology, the ash is the World Tree Yggdrasil, whose roots reach into the lower world and whose trunk connects the circles of existence, and beside every World Tree there is a spring or pool from which a river flows. Whether by coincidence or by the deep patterning of human imagination, the ash at Thames Head stands where the river emerges from darkness into light, a metaphor of birth and death that has drawn pilgrims and poets for centuries. The river begins in mystery, flows through the heart of a kingdom, and empties into the sea that leads to the edge of the world, and at every stage it has been treated as a god, a boundary, a grave, and a gateway.