Sometime during the First Age, Beleg and his friend Mablung were the only Elves of Doriath to fight with the Union of Maedhros in the Nirnaeth Arnoediad, as King Thingol would have nothing to do with the war. Together with Mablung he was one of the great captains of the Sindar, and was part of the Hunting of the Great Wolf Carcharoth during the trials of Beren Erchamion and Lúthien.
Later, he became a good friend and brother-in-arms of Túrin Turambar. When Túrin left Doriath, Beleg got permission from the King to follow him into exile and bring him the kings pardon, wielding the black sword Anglachel that Eöl had forged and given to Thingol in tribute. Beleg found the band of outlaws Turin was leading while Turin was away scouting, and was tortured by the outlaws until Turin found and released him. Unable to persuade Turin to return to Doriath with him, Beleg returned to King Thingol and gained permission to join Turin. He went into the wilds again and found Turin on Amon Rûdh.
Together with Túrin, wearing the Dragon-helm of Dor-lómin, he became for a while a captain against the Dark Lord Morgoth. When Túrin was captured by Morgoth, Beleg was grievously wounded, yet with the knowledge and strength of an elite elven warrior, he recovered and followed the orcs. He met the escaped thrall Gwindor in Taur-nu-Fuin and together they rescued Túrin, but as Beleg cut the bonds from Túrin's wrists and ankles he slipped, accidentally cutting Túrin with his sword. Túrin awoke and did not recognize Beleg in the darkness, assuming the shape bent over him with a blade was an orc and that he was to be tortured again. In a sudden rage of self-defense, he wrested the sword from Beleg and killed him.