Rúmil was possibly born in Valinor or perhaps one of the firstborn. He thrived well in Valinor being the first to invent the art of writing down words with pen or brush on many different mediums: he invented the alphabet called Sarati (each letter being a Sarat). This alphabet was later expanded and perfected by Fëanor as the Tengwar. Rúmil was also a skilled linguist, and when the Teleri finally arrived in Valinor he was first to discover just how the Telerin language had changed from Common Eldarin compared to Quenya. He became part of an elvish culture of loremasters known as the Lambengolmor.
When the Ñoldor were determined to return to Middle-earth in pursuit of Morgoth, Rúmil was one of the Ñoldor who refused the summons of Fëanor, and he remained in Tirion as one of the Ñoldor of Finarfin, where he presumably remains still.
In The History of Middle-earth series, Rúmil is additionally given as the original author of much of the work which is included in The Silmarillion: the Ainulindalë, Valaquenta and Annals of Aman are by his hand. He is also given as the author of the Lhammas and Lhammasethen, as well as the Ambarkanta.