Mentor is the son of Herakles and Asopis who is one of the daughters of King Thespius. During his stay in the court of King Thespius, Herakles impregnated 49 out of the 50 daughters of Thespius including Asopis who he lay with one night. The result of that particular encounter was Mentor.
In his old age Mentor was a friend of Odysseus who placed him in charge of his son Telemachus, and of Odysseus' palace, when he left for the Trojan War.
It is said that when Athena visited Telemachus she disguised herself as Mentor to hide from the suitors of Telemachus's mother Penelope. In this disguise she encouraged Telemachus to stand up against the suitors and go abroad to find out what happened to his father. When Odysseus returned to Ithaca, she appeared briefly in the form of Mentor once again at Odysseus' palace.
"by Asopis he [Herakles] had Mentor" - Apollod. Biblioteca 2.7.8