Éomer was the oldest son of Théodwyn, sister of King Théoden of Rohan. He had a sister Éowyn. After they were orphaned Théoden adopted them as his own children.

Éomer was a Marshall of the Mark, and was a strong leader of Men. His éored hunted down the Uruk-hai that had taken 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 granted them two horses.

Éomer returned to Edoras, and reported on meeting the Ranger and friends. However, under the poisonous advice of Gríma Wormtongue, he was arrested.

He was set free when Theóden King recovered from the influence of Saruman, and was with his King at the battle of Helm's Deep, where he and his éored drove the Orcs and Dunlendings of Saruman into the walls of the Hornburg and in the Huron forest, where they were killed.

Afterwards he rode with Théoden King, 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.

In the year 3019 at the Battle of the Pelennor Fields Théoden King was killed, and Éomer became his successor. He stayed long behind in Minas Tirith to help Aragorn rebuild his Kingdom, before returning home to be crowned King himself.

Éomer renewed the Oath or Eorl for Aragorn after Aragorn had been crowned King Elessar of the Reunited Kingdom, and Éomer had been crowned King of Rohan, swearing everlasting friendship between Rohan and Gondor. (And legalizing Cirion's grant of Calenardhon to the Éothéod.)

He became known as Éomer Éadig, or "the Blessed", because during his reign Rohan recovered from the hurts of the War and became a rich and fruitful land again.

Éomer had met Lothiriel, daughter of Prince Imrahil of Dol Amroth during his stay in Gondor, and they were wed. She bore him a son Elfwine, who succeeded his father in the year 63 of the Fourth Age.

In the The Lord of the Rings movie trilogy by Peter Jackson, Eómer is exiled by Gríma before meeting Aragorn, and is an outcast until he arrives at Helm's Deep, summoned by Gandalf.