In the maps of the Second and Third Age it looked like the Ered Mithrin were but a northern arm of the Hithaeglir, but in truth this mountain range was far older, stemming from the creation of Arda, whereas the Misty Mountains had not been raised until after the Years of the Lamps.
Where the Ered Mithrin met at their western end with the Hithaeglir lay Mount Gundabad, an ancient Dwarven holy site. The stretch of mountains west of the Hithaeglir which still formed one range with the Grey Mountains was known as the Mountains of Angmar, another remnant of the Ered Engrin.
The eastern end of the Ered Mithrin was split in two branches, and in between lay the Withered Heath, where dragons still bred. After that was a long gap, until the Iron Hills continued the old line of the Iron Mountains again. Erebor, the Lonely Mountain, was not part of either range.
Of old the Grey Mountains had been mined by Dwarves of Durin's Folk, but by the Third Age all Dwarven strongholds had been abandoned or raided by dragons. Its sole purpose now seemed to be to divide Forodwaith from Wilderland.
Since no maps of the entire world exist after the First Age, it is unknown if this mountain line still existed in the Third Age, whether reduced or intact.