The First Marshal of the Mark typically commanded the Kings Lands and was in charge of the great Muster of Rohan.

During the time of the War of the Ring, Théoden had no First Marshal. He had become king at a young age and as he was an excellent warrior and horseman. In the event of War, he would have commanded the Muster of Rohan himself - which would have included a muster of all able-bodied riders.

The other duties of the First Marshal fell to a lower marshal named Elfhelm. He had the power to call a Muster of Edoras.
Encyclopedia entry originally written by Nienna-of-the-Valar