Éomer was the nephew and adopted son of King Théoden of Rohan. Upon Theoden's death in T.A. 3019, he became the eighteenth King of Rohan, and the first of the Third Line. He ruled until F.A. 63, orchestrating Rohan's growth throughout an era of unprecedented peace and prosperity. Éomer was born in Edoras in T.A. 2991. His father, Éomund, was Chief Marshall of the Mark until he was ambushed and slain by Orcs in T.A. 3002. Since Éomer's mother, Théodwyn, died of grief in the King's household soon afterwards, King Theoden adopted the eleven year-old boy and his older sister Éowyn. Éomer was groomed for a position of great honor and responsibility and received the finest training available to the Horse-lords of the Riddermark, but Prince Théodred was Theoden' s sole true son and therefore held the status as heir to the throne. When the War of the Ring began in T.A. 3018, Éomer held the title of Third Marshall of the Mark (behind King Théoden and Prince Théodred). He was charged with overseeing the East-mark and resided in the capital of Edoras. There, he met and befriended Aragorn Elessar and challenged the rise of Grima Wormtongue. Unfor- tunately, Eomer's position in Théoden's court eroded when the King fell under Grima's sway and, at one point, the king ordered his foster-son imprisoned for allowing strangers to roam in the East-mark. Gandalfs intercession ended Grima's grip on Theoden, though, and Éomer reassumed the King's favor. During the War of the Ring, Éomer fought at Helms Deep, Pelennor Fields, and Morannon. His valor matched his might, and he distinguished himself throughout the conflict. Coupled with the loyalty he showed throughout his struggle with Grima, he made Théoden proud, and the King's love for Éomer grew throughout the bitter conflict. With Prince Théodred's death at thee First Battle of the Fords of Isen, Éomer became the natural heir to the throne (although he did not know he would be Lord of the Riddermark until he assumed the role). Éomer became the new King when Théoden transferred the royal banner at the Battle of Pelennor Fields — only moments before Théoden died from the mortal wound delivered by the Lord of the Nazgûl. After the War, Éomer renewed the Oath of Eorl with King Aragorn Elessar and campaigned with the Dúnadan King throughout the early Fourth Age. He married Lothíriel, the daughter of Prince Imrahil of Dol Amroth, and his only child and son, Elfwine the Fair, succeeded him upon his death in F.A. 63.
Eomer stood 6'9", which was tall even by Rohir standards. He owed his height to the blood he inherited from his mother Théodwyn. Théodwyn was the daughter of King Thengel (Rohan's sixteenth monarch) and Morwen of Lossarnach, a Dunadan woman from the line of the Lords of Dor-en-Ernil who Thengel met during his long stay at Minas Tirith. Éomer still looked like a Rohir, however: blond-haired and fair, strong yet graceful, and stern in countenance.