Phaedo was from a noble family in Elis. In the war of 401-400 between Sparta and Elis the city was defeated. He was captured and sold into slavery at Athens to the owner of a house of male prostitution, where for his beauty he became quite famous.

It was as a male prostitute that Socrates first found him, indicating that Socrates frequented such houses. Finding Phaedo not only beautiful but intelligent and spirited (kalos kai agathos) Socrates eventually persuaded his friends to ransom him so that he could be free and study philosophy.

The handsome young man was thus set free and became one of the most devoted students of Socrates, who conceived a warm affection for him. Also Phaedo become enamored of Socrates and his teachings. He was much younger than Socrates, he was about eighteen years old at the time of Socrates' execution. It appears that he was intimate with Cebes and Plato, and he gave his name to one of Plato's dialogues.

As told in Plato's Phaedo it was this young man with whom Socrates had his last dialogue on the immortality of the soul just before he is forced to drink hemlock for "corrupting Athenian youth". This so-called "corruption" didn't have to do with sex, it had to do with "ideas".

Shortly after the death of Socrates, Phaedo returned to Elis where he opened his own school of philosophy. Elis was approximately one hundred miles west of Athens on the Peloponnesian Peninsula. There he composed his own dialogues, focusing on the field of ethics. The doctrines of Phaedo are not known, nor is it possible to infer them from the Platonic dialogue. His writings, none of which are preserved, were also in the form of dialogues.

According to one interpretation, Socrates and Phaedo were lovers. In ancient Athens, as well as other Greek cities, sex between adult men and young males who had reached puberty was culturally acceptable. It was often part of a mentoring program intended to educate the male youth by exposing them to the activities in which they were expected to participate. These activities included politics, economics, training for war, and sex.

Seneca has preserved one of his dicta (Epist. 94. 41), namely that "one method of acquiring virtue is to frequent the society of good men".