Mitherion, along with his brother Mithgelion, were Sindar Elves who, although setting out on the journey towards Valinor, decided to stay behind in the end. Both journeyed from the shores of Middle Earth to lands further North and became part of the Mithrim, establishing their home in the lands of Nevrast.

When the Noldor returned to Middle Earth, Mitherion and his brother initially observed warily and did not approach, fearing what their arrival might entail. Soon, however, they began to mingle with the settlers, and ultimately accepted Turgon as their King, moving to his capital of Vinyamar.

When Turgon announced the completion of the hidden city of Gondolin, the brothers once again moved to follow their King and settled in the Hidden City, serving as soldiers within the host of the Gondolindrim. When the Eagles of Thorondor arrived in the City, bearing two of the House of Hador with them, Mitherion grew curious regarding the Secondborn of Iluvatar. He learned much from them, and they from the Gondolindrim. When the brothers left Gondolin a year after, Mitherion was sorrowful, but kept both in his memory.

Several years after came another Man, and Mitherion recognized him as one from the House of Hador, and grew joyful when he announced himself the son of Huor. He swore fealty to the House of the Wing and worked in the service of Tuor for the remaining of his days in Gondolin. His brother, Mithgelion, was of the House of the Fountain.

When Gondolin was besieged by the forces of Morgoth, Mitherion fought alongside others under Tuor in defending the walls alongside the House of the Fountain. Ultimately, both Houses had to retreat back to the Square of the King, where they fought to give evacuees time to head towards Idril's secret path. They joined the evacuees soon after, and settled in the Havens built on the Mouth of Sirion.

In the Second Age, both Mitherion and Mithgelion served under Gil-galad, High King of the Noldor, and both have resided in Imladris ever since.