(Drawn largely from the Tolkien Gateway article of the same name, worth revisiting as it is currently receiving additional attention.)
Mirkwood (S: Taur-nu-Fuin, literally “Forest under Shadow”1, also S: Taur-e-Ndaedelos or the Forest of Great Fear) is the great forest that dominates the region of Rhovanion. It has only become known by these names in the latter part of the Third Age, since some time after TA 1000, having previously been called Greenwood the Great (S: Eryn Galen).
The forest once formed part of the vast primaeval woodland which covered most of Middle-earth at the outset of the First Age before the rising of the Sun. The Eldar passed through the area on their journey into the West, and Greenwood the Great was first populated at this time by the Nandor elves who were unwilling to cross the Misty Mountains and settled in the wooded valleys of the river Anduin. They thrived were joined by wandering Avari elves, the combined folk becoming known as Silvan or Wood-elves.
The Old Forest Road was constructed very early in Greenwood’s history, probably by the Dwarves to carry traffic between their eastern and western holds.
Greenwood first appeared to history in recognisable form on the arrival of the Sinda lord Oropher and his founding his hold at Amon Lanc at the beginning of the Second Age. Oropher was accepted as the leader of the Wood-elves of Greenwood, becoming the king of the Woodland Realm.
Presumably by this time Men had also settled in and around the forest in small numbers.
The first millennium of the Third Age probably saw the creation of the East Bight by men living in the eastern eaves of the forest. These men may have formed part of the Kingdom of Rhovanion led by Vidugavia. Men, such as the Éothéod, and hobbits, also lived in the vale of Anduin and were likely responsible for the retreat of the forest’s western border.
At the beginning of the Third Age Thranduil succeeded his father Oropher as king of the Woodland Realm. Probably as a result of massive losses at the Battle of Dagorlad the Silvan population of Greenwood was diminished and became mainly concentrated in the hills then known as Emyn Duir. This included the abandonment of Amon Lanc, and around the turn of the first millennium Sauron, under the guise of the ‘Necromancer’, returned to Middle-earth and in T.A. 1050 built a fortress there. The hill and the fortress together become known as Dol Guldur, the “Hill of Sorcery”.
Sauron’s arrival caused a darkening of Greenwood, and it is at this point it became known as Mirkwood. The children of Shelob, giant spiders, as well as bats and orcs in Dol Guldur’s service occupied the forest and it became thicker, darker and covered in cobwebs.2
This caused the Silvan population of Mirkwood to retreat even further, residing apparently exclusively in Thranduil’s halls at the eastern end of the Forest River. The ancient Old Forest Road was abandoned by men and dwarves alike, with a new but seldom used path being made further from Dol Guldur. The hobbits near the forest’s western borders migrated away.
The shadow over Mirkwood was lifted in T.A. 2941 when the White Council, prompted by the wizard Gandalf’s discovery of his true identity, drove the Necromancer, Sauron, from Dol Guldur. Gandalf also instigated the Quest for Erebor which resulted in the slaying of Smaug in the same year. The combination of these two events allowed the re-established kingdoms of Erebor and Dale, as well as the Woodland Realm and a confederacy of Woodmen led by the Beornings to flourish for a brief period.
1 This must be presumed to be a deliberate reference to the forest of the same name in Dorthonion, in the Beleriand of the First Age, which was destroyed with the breaking of that land in the War of Wrath at the end of the Age.
2 J.R.R. Tolkien, The Hobbit, “Flies and Spiders”
3 J.R.R. Tolkien, The Hobbit, “Map of Wilderland”