The Third Marshal of the Mark typically commanded the East-mark and his home base was at Aldburg in the Folde. He had the power to summon a Muster of the East-mark.

During the War of the Ring, Éomer son of Éomund served as the Third Marshal as his father had before him.

After the healing of Théoden by Gandalf and the death of Theodred, the situation changed. When the Full Muster of Rohan was made in Harrowdale before the Battle of the Pelennor Fields, Théoden was able to again take command of his men and Éomer was essentially the First Marshal of the Mark, ready to take command if Théoden should die, though the title was not used. Elfhelm became a Marshal of the Mark and he led the first éored of the Muster of the East-mark. Grimbold commanded the Muster of the West-mark instead of Erkenbrand because Erkenbrand was charged with the defense of Rohan.

When Théoden fell on the Pelennor Fields, Éomer became the king and also the commander of his own Muster of Rohan. Grimbold fell on the Pelennor and Elfhelm was made the lieutenant of Éomer. When the new king went to the Black Gate, Elfhelm was left in charge of the Rohirrim who were left behind in Gondor.