Thorondor was sent by Manwë, King of the Valar, to watch over the Ñoldor after they arrived in Beleriand. The Eldar first encountered him when he helped Fingon rescue Maedhros from imprisonment from Thangorodrim, upon which he had apparently made his home for a short time. After that, Thorondor and the rest of the Eagles settled in the Crissaegrim. When Gondolin was built, Thorondor became the ever vigilant guardian of the city. He rescued Fingolfin's body from defilement after Morgoth killed him, giving the Dark Lord a scar on his face. He and two other Eagles rescued Beren and Lúthien when Angband was aroused.
When Gondolin fell, Thorondor rescued the survivors. During the War of Wrath, Thorondor and Eärendil led the Eagles in battle with the Dragons, and may have fought with Ancalagon the Black. In Tolkien's writings, Thorondor is not mentioned after the War of Wrath. It is believed he returned to Valinor, although his descendants remained behind. Gwaihir became the Lord of Eagles in Middle-earth after Thorondor's departure.