Born in Gondolin in FA379, Belethrien was serious and scholarly from a young age. She was a member of the House of the Pillar in Gondolin and acted as an advisor and junior diplomat. After the Fall of Gondolin Belethrien joined other survivors of the city in the House of Vanimar.

For many years after the Fall of Gondolin, Belethrien retreated from the world into scholarship.

In SA1296 Belethrien heard tales of the great new scholarship and craft of the elves of Eregion and  went to reside there. Between SA1304 and SA1319 she wrote a lengthy treatise on minor ring-lore.

In SA1697, Belethrien fled the invasion of Eregion with the host of the Eldar under Elrond Half-Elven, She joined others of that host in the establishment of the refuge of Imladris at the feet of the Misty Mountains. It was at this time that she experienced a spiritual crisis, of which she will not speak, brought on by twice witnessing the destruction of a land she had called home.

After the siege of Imladris was lifted and the forces of the enemy driven back, Belethrien travelled to Lindon to take counsel there. She then disappeared for a number of years, not seen in any haven or refuge of the Eldar.

It is known that she spent time on the island of Numenor but departed from that place in SA2961.

By this time, all her focus had turned to study of the ways of the Enemy and seeking to understand how his influence acted upon the different peoples of Middle Earth. She spent considerable time in Lindon and Imladris, taking counsel from the Wise.

Belethrien rallied to the banner of High King Gil-galad during the War of the Last Alliance, taking a role first as a battlefield healer and then an advisor on the composition and behavioural characteristics of the forces of the Enemy, aiding military companies in strategic decisions. She dwelt with the besieging army at the gates of Barad-dûr for seven full years.

Belethrien spent the early part of the Third Age in Imladris, before leaving suddenly, not seen again in Eriador until the rise of the Enemy in the East.