Mithrim formed a part of Hithlum, and was the south-western corner of it, bordering Dor-lómin to the east, from which it was divided by the Mountains of Mithrim. Mithrims climate was the same as Hithlums the air was cool and the winters was cold but it was a fair land.
The area was home to a great lake, the Lake of Mithrim, which was the body of water north of Beleriand where the Noldor first dwelt in Middle-earth: the Sons of Fëanor on the northern shore and Fingolfin's host on the southern shore.
The Ñoldor dwelt here for a while until their feud was healed, and they removed to other lands. Mithrim was also home to Sindarin Elves, who soon mingled with the Ñoldor after they had learned Sindarin.