Mormon is the one who tells the main story in Mosiah

The main story in the book of Mosiah is told in third person rather than in first person as was the custom in the earlier books of the Book of Mormon. "The reason for this is that someone else is now telling the story, and that `someone else' is Mormon," wrote Daniel H. Ludlow in A Companion to Your Study of the Book of Mormon.

Mormon, whose father was also named Mormon (Morm. 1:5), was the last great prophet-general of the Nephite nation. He is better known as the compiler of the records of his people."With the beginning of the book of Mosiah we start our study of Mormon's abridgment of various books that had been written on the large plates of Nephi (2 Ne. 5:8-12)," wrote Ludlow.

"The book of Mosiah and the five books that follow – Alma, Helaman, 3 Nephi, 4 Nephi, and Mormon – were all abridged or condensed by Mormon from the large plates of Nephi, and these abridged versions were written by Mormon on the plates that bear his name, the plates of Mormon.

"These were the same plates that were given to Joseph Smith by the Angel Moroni on Sept. 22, 1827."