Éomer was third Marshal of the Riddermark, and was a strong leader of Men. His éored hunted down the Uruk-hai that had taken the Hobbits Meriadoc Brandybuck and Peregrin Took as they camped near Fangorn forest, slaying them all. He never saw the Hobbits, as they fled during the battle. Afterwards he met Aragorn, Gimli and Legolas in the plains of Rohan, and he lent them two horses, Hasufel and Arod.
He was set free when Théoden was released from the influence of Saruman. He was with his king at the battle of the Hornburg, where he and his éored, led by Théoden and Aragorn, drove Saruman's army of Orcs and Dunlendings from the walls of the Hornburg. As they fought, Gandalf arrived with Erkenbrand, a leader of the Rohirrim driven from Helm's Deep during the Battles at the Fords of Isen, and the troops that Gandalf had gathered from the chaos of the Battle of the Fords of Isen. Dismayed at this unexpected attack from the side, Saruman's Orcs were driven into the forest of Huorns, where they were destroyed; the men of Dunland were forced to surrender.
Afterwards, Éomer rode with Théoden, Aragorn, and Gandalf to Isengard, where they confronted a trapped Saruman. Éomer rode with his king to the Battle of the Pelennor Fields, and fought bravely for Rohan and Gondor.
Théoden was mortally wounded in the battle, and named Éomer as his successor. Believing his sister Éowyn to be slain by the Witch-king, he threw himself and his men wildly at the enemy. Surrounded and outnumbered by the Orcs, he was rescued when Aragorn arrived from Pelargir, thereby fulfilling his prediction that they would meet and fight together in battle. He later accompanied Aragorn to the Gates of Mordor and the Field of Cormallen. After Théoden's funeral, he stayed in Minas Tirith to help Aragorn rebuild his own kingdom (for the latter was crowned King Elessar of the Reunited Kingdom), before returning home to be crowned King of Rohan himself. Éomer was the eighteenth king of Rohan, and the first king of the Third Line (Théoden's death ended the Second Line of kings; Helm Hammerhand's death ended the First).
Éomer renewed the Oath of Eorl for Aragorn after both of them had been crowned, swearing everlasting friendship between Rohan and Gondor (and confirming Cirion's grant of Calenardhon to the Éothéod, the ancestors of the Rohirrim.)
He later became known as Éomer Éadig, or "the Blessed", because Rohan recovered from the hurts of the war and became a rich and fruitful land again during his reign.
Éomer had met Princess Lothíriel, daughter of Prince Imrahil of Dol Amroth, during his stay in Gondor, and they were wed. She bore him a son, Elfwine the Fair, who succeeded his father as the King of Rohan after Éomer's death in the year . Merry and Pippin had retired from the Shire to Rohan at Éomer's request and were with the old King when he died.
In Peter Jackson's The Lord of the Rings film trilogy, Éomer was played by New Zealand actor Karl Urban. In The Lord of the Rings: The Two Towers, he is exiled by Gríma before meeting Aragorn. As an outcast, he leads a troop of riders loyal to Théoden.
In both the Bakshi and Jackson's versions, he arrives at the climax of the "Battle of Helm's Deep", accompanied by Gandalf (although the animated film does not single him out at Helm's Deep). In this sense, his character has been combined with the character of Erkenbrand, who, in the book, is the one with whom Gandalf returns to Helm's Deep.
Éomer's actions in Jackson's adaptation of The Return of the King did not significantly depart from those in the book, save for a few scenes (such as Éomer letting out a defiant cry at the approaching Corsair ships during the Battle of the Pelennor Fields, only to realize the ships have been captured by Aragorn) that were omitted for time.