Born in Ladros, Andreth grew up to be a wise person in thought and learned in the lore of Men. In her youth, she dwelt in the house of Belemir in Ladros, where she had learned the lore of the House of Marach from Adanel. Despite her love for the elves, Andreth was given gift of been ageless in 430 1A.
Andreth was deeply in love with Aegnor Aikanáro, son of Finarfin. The love was returned but never fulfilled, since Aegnor departed from motives of wisdom, as he felt his coming death, thus they both remained unwed and childless. She had a close friendship with Aegnor's brother Finrod Felagund, who often visited her during the Siege of Angband to converse and debate with her on the matters of Elves and Men. One such conversation was written down and later known as Athrabeth Finrod ah Andreth, where she and Finrod discussed differences between Elves and Men.
In 455 1A, Andreth fought valiantly in the Battle of Sudden Flame when Morgoth broke the Siege of Angband. She was very strong and brave when she faced the enemy, and attempted to face Morgoth. However, she was immediately slain by the Dark Lord.