Damis was born in Nineveh, the ancient Assyrian city on the Tigris River that had been destroyed by the Medes and Babylonians in 612 BCE but which remained an important settlement in the Parthian and Roman periods. He was a young man of good family, educated in Greek philosophy and eager for wisdom, when he first encountered Apollonius of Tyana. The meeting took place in the town of Nineveh itself, or in some accounts nearby, around the year 43 CE. Apollonius was already famous as a philosopher, a vegetarian, a celibate, and a worker of miracles who claimed descent from the ancient sage Pythagoras, the philosopher who had founded a religious brotherhood in southern Italy and who believed in the transmigration of souls. Damis was captivated. He asked to become the philosopher's disciple, and Apollonius accepted him. From that day forward, Damis would not leave his master's side for more than thirty years.

The journey began in earnest when Apollonius resolved to travel to India to study with the Brahmans, the wise men of the East who were said to possess knowledge far beyond anything known in the Mediterranean world. Damis accompanied him every step of the way. They crossed the Euphrates, travelled through Mesopotamia and Media, ascended the Caucasus Mountains, and descended into the Indus valley. Damis recorded everything: the customs of the Parthians, the gymnosophists who studied naked in the Indian sun, the elephants and tigers of the eastern forests, and above all the conversations between Apollonius and the Brahmans, who tested the Greek philosopher with riddles and philosophical challenges. In Philostratus's telling, Damis's memoirs were the source for these detailed accounts, the only eyewitness record of a Greek philosopher's encounter with Indian wisdom.

After India, the companions returned to the West. They visited Babylon, where Apollonius conversed with the Magi, the priests of the Persian fire-cult. They travelled to Athens, where Apollonius debated with the Stoics and the Epicureans. They went to Rome, where they fell afoul of Nero, the emperor whose madness had made him the most dangerous ruler in the Mediterranean, and later of Domitian, the emperor who ruled from 81 to 96 CE with a combination of military efficiency and paranoid cruelty. Domitian arrested Apollonius and put him on trial for magic and sedition. Damis was imprisoned alongside his master, and he recorded the philosopher's defiant defence before the emperor, a speech so eloquent that Domitian, unable to find a pretext for execution, was forced to release him.

The most mysterious episode in Damis's memoirs concerns the end of Apollonius's life. The philosopher had travelled to Ephesus, where he suddenly called the citizens together and announced that a great tyrant had died at that very hour. The tyrant was Domitian, who was indeed being assassinated in Rome at the moment Apollonius spoke. Later, Apollonius entered the temple of Dictynna on Crete and simply vanished. Some said he had been assumed into heaven like the disciples of Pythagoras. Others said he had never died at all but continued to wander the earth in secret. Damis, according to Philostratus, recorded these events faithfully and then retired to write his memoirs, which he left as a testament to the life of the man he had followed across half the world.

The historical existence of Damis has been debated since the Renaissance. Philostratus claimed that he received the memoirs from Julia Domna, the Syrian empress who was the wife of Septimius Severus and the mother of Caracalla. She was a patron of philosophy and literature, and Philostratus said that she had commissioned him to write the Life of Apollonius from the notes of the Assyrian disciple. But modern scholars are divided. Some, following the French scholar Petzold and the German philologist Solmsen, argue that Damis was a genuine historical figure and that his memoirs were a real source, however much Philostratus embellished them. Others, including the influential scholar Graham Anderson, contend that Damis was a literary fiction invented by Philostratus to give his philosophical romance the authority of an eyewitness account. The debate remains open, and Damis remains a ghost at the edge of history, the disciple who may or may not have existed but whose name is inseparable from the most fascinating biography of the ancient world.