Cyrene was born in Thessaly, the northern part of Greece, to Hypseus, the King of the Lapiths. Since Hypseus' grandfather was Oceanus and his father a river spirit, Cyrene was not a full mortal herself.
Cyrene grew up dreaming of war and conquest, and wanted to be a great warrior like her father, who put her in charge of guarding the cattle and the sheep. She took to this job quickly, slaying any predator (mortal and supernatural alike) who posed a threat to the herds and flocks, and proved to be such a deadly efficient guardian that all the cattle and the sheep completely entrusted her with their safety, and she had no qualms with driving them to dangerous places so she could hunt and kill more dangerous monsters.
Later on, her extraordinary hunting skills caught the attention of Artemis, who gave her two fine hunting dogs as gifts and tried to recruit her to join her Hunters, but Cyrene respectfully declined the goddess as she still wished to be married one day. With her new hunting dogs, Cyrene became even more deadly a warrior, to the extent where all the local predators steered clear of her and the animals under her watch. One day, she spotted a lion stalking her flock and, despite her lack of weapons at that point in time, she still confronted the lion and managed to strangle it to death.
Unknown to Cyrene, Apollo, who had heard of her from Artemis, had been watching her fight with the lion, and desired to take her as his lover. He appeared to Cyrene in all his majesty, and courted her with praises of her virtues and even the promise of a kingdom of her own to rule if she would go with him. Intrigued by Apollo's handsomeness and sweet words, she agreed to go with him, and took her faithful hunting dogs with them as well.
Apollo took Cyrene to the northern coast of Africa (known in the modern times as Libya), where they enjoyed an intense and passionate love affair. Thanks to her lover's Oracles spreading the word of a new queen, an entire colony of Greeks came to thrive in her new home. They built a city that was named after her, and dedicated their biggest and most important temple to Apollo.
Apollo and Cyrene also had two sons: Aristaios (who later became a minor deity of beekeeping and cheese-making) and Idmon (a seer who still chose to die a hero's tragic death despite his having foreseen it). Her city became the first and most important Greek colony in Africa, and - though she was unaware of it then - was to survive through most of the Roman Empire. In spite of all this, Cyrene grew discontented with her life - her hunting dogs passed away, her children grew up, and Apollo's visits became increasingly seldom, as it was custom for gods to tire of their mortal lovers in time. Hence, she decided to go back to Thessaly one more time to visit her childhood friends and see if her father was alive.
When she finally reached Thessaly, she learned that things had completely and irrevocably changed from what she had remembered: her father had passed away, and the new King of the Lapiths did not want anything to do with her. Her friends had either married and did not even recognise her, or had died from the harsh life the Lapiths led. As she roamed the old paths where she used to herd sheep, Cyrene encountered Ares himself, and engaged him in a battle that unexpectedly concluded with their becoming passionate lovers. They stayed together for many months, and even had a son, Diomedes, who became the King of Thrace.
Cyrene eventually returned to her city on the African coast, and found Apollo waiting for her at the exact same spot where they had first landed many years ago. Despite her romance with Ares, Apollo assured her that she owed him no explanations, and acknowledged his faults of being an increasingly inattentive lover to her as well as having taken her away from her native land. However, he still observed that their romance was at an end, to which Cyrene was relieved as she was ready for a peaceful and quiet life.
After assuring Apollo that her city was where she belonged, he transformed her into the patron naiad of her city, promising that her new state of existence guaranteed her exceptional longevity, peace, and good health. With that, Apollo's and Cyrene's romance came to an official end, though it would seem that they still parted on amiable terms.
As Apollo had promised, Cyrene the Naiad lived a very long life, and she later left her Greek colony to live full-time with the other naiads in the river. However, she would occasionally rise to offer advice to her friends and family - once, when her son Aristaios lost his bees, she helped him find them again.
In Percy Jackson's Greek Heroes, Cyrene was described to have long dark hair, graceful limbs that were the color of polished bronze in the sunlight, and a face that was beautiful and serene even in the midst of combat. When Apollo saw her for the first time, he also noted that she reminded him of a Goddess of War.
- Prowess in Battle: Cyrene was a formidable and skillful warrior, being an expert in the spear, the sword, and the bow. Her "war against wild animals" resulted in the sheep and cow under her watch having a complete trust in her to preserve their safety against any and all threats, and there was even the possibility of her having "killed every predator within a fifty-mile radius". She was also able to hold her own against Ares himself in battle.
- Hunting: Cyrene was an expert in hunting and killing vicious predators, even those who were much larger or supposedly far stronger than her. In fact, she was such an accomplished huntress that the animals under her watch never worried about nearby predators due to their utter trust in her, and Artemis once tried to recruit her to join her Hunters. Though Cyrene declined, the goddess still retained a favorable opinion of her, and praised her skills to Apollo.
- Tracking: Cyrene was an expert in seeking out any and all potential threats to the animals that were under her watch, which is a skill she developed from her constant patrolling of the hills, the wilderness, and other dangerous places. Even after she became a naiad, she was shown to have retained this skill - she once helped her son, Aristaios, to find his bees when he lost them.
- Archery: Artemis described Cyrene as being "amazing with the bow", which shows that she was an expert archer.
- Superhuman Strength (possibly): It was possible that Cyrene possessed superhuman strength, given how she fought and killed a large male lion without any weapons - she was able to pick it up by its back legs and toss it into a boulder, and finally strangle it to death.
- Longevity: Cyrene was said to live a very long life after becoming a naiad, though it was never specifically stated or confirmed if she had become truly immortal.
It was also possible that Cyrene possessed other special powers after she became a naiad, as naiads are spirits of nature who derived supernatural powers from their particular provinces. However, this was never specifically confirmed or revealed.