The appendices of Return of the King explain that Théoden was the only son of King Thengel and Morwen of
Lossarnach (a region of Gondor). He was the second-born of five children, and the only boy. Théoden was closest to his youngest sister, Theodwyn. He was born in Gondor, where his family lived until Thengel became king of Rohan.
He became king after the death of his father. Théodwyn lived with him in Edoras. He married Elfhild, but she died giving birth to their son, Théodred. After Théodwyn and her husband Emond also died, he adopted their children, Éomer and Éowyn.
In his prime, Théoden was a strong and vital king, highly respected by his subjects. As with other Men of the Riddlemark, Théoden was a skilled horseman.
He acted as the First Marshal of the Mark after the death of Éomund, who had filled that position; as First Marshal he commanded the Muster of Edoras (Théodred and Éomer were respectively the Second and Third Marshal). His sword was called Herogrim.