Thranduil was one of many Sindar who travelled eastward from Lindon at the beginning of the Second Age. He eventually ended up in Greenwood the Great, where Silvan Elves of Nandorin descent lived. His father Oropher was taken by them as lord and founded the Woodland Realm with the capital at Amon Lanc.
The few Sindar who had come with Oropher and Thranduil were soon merged with the Silvan Elves, adopting their language and taking names of Silvan form and style.
In S.A. 3434 Thranduil followed his father, and some thirty thousand lightly armed Elves in the War of the Last Alliance. In the Battle of Dagorlad Oropher was slain with the greater part of his people but his son survived. After the Siege of Barad-dûr in S.A. 3441 when Sauron was defeated, Thranduil led the remainder of his people north back to the Woodland Realm, where he was crowned king.
With the return of Sauron around T.A. 1050 southern Greenwood became dangerous and was renamed Mirkwood. Creatures like great spiders came to dwell in Mirkwood and Thranduil’s folk retreated to the northeastern corner of Mirkwood, where they fortified themselves near the Forest River.
Thranduil first appears in The Hobbit as the Elvenking, when Bilbo and the Dwarves enter his realm in the northern part of Mirkwood. The Dwarves are captured by Thranduil’s guards and locked in his dungeons when they refuse to divulge their intentions. The Dwarves were rescued by Bilbo, who had remained in hiding with his use of a magic ring of invisibility.
After the death of the dragon Smaug, Thranduil along with people of Lake-town demanded a share of the treasure of Erebor. War with the Dwarves was averted by the arrival of a goblin/warg army and the ensuing battle.
Thranduil was the father of Legolas, a supporting character in The Lord of the Rings. The gradually-established friendship between Legolas and the Dwarf Gimli, the son of Glóin of The Hobbit, helps to reconcile Thranduil’s people and the Dwarves. The appendices to The Return of the King note that Thranduil withstood attacks by Sauron during northern battles of the War of the Ring, meeting with Celeborn and his people to together destroy Dol Guldur and cleanse Mirkwood of Sauron’s taint of evil. Also, Legolas and the Silvan Elves later worked together with Gimli and the Dwarves to rebuild and improve Minas Tirith, capital city of Gondor, the realm of their mutual friend Aragorn. The last time Thranduil was mentioned was soon after Sauron’s final defeat.