According to Daniel Ludlow, the word "Mormon" is used chronologically for the first time in Mosiah 18:4. Although the word Mormon appears earlier than this in the Book of Mormon it has always referred to the name of the great prophet, historian, and military leader who lived several hundred years after the time of Christ. . . . According to Ludlow, the place which was called Mormon "received its name from the king"; therefore, it was apparently named after a king called Mormon. [Daniel Ludlow, A Companion to Your Study of the Book of Mormon, pp. 188, 80]
According to Joseph Allen, Lake Atitlan is precisely the kind of place that the riotously-living King Noah would favor as a resort for himself and his priests. After all, King Noah named the place Mormon (Mosiah 18:4). [Joseph L. Allen, Exploring the Lands of the Book of Mormon, p. 325]
“A Place Which Was Called Mormon Having Received Its Name from the King”
In Mosiah 18:4 we read that those who believed Alma "did go forth to a place which was called Mormon, having received its name from the king, being in the borders of the land having been infested, by times or at seasons, by wild beasts."
John Tvedtnes notes that on the surface, one might suspect that it was King Noah, who reigned in Nephi in Alma's time, who had given the name Mormon to the site. But there is another possibility. In an unpublished paper, Charles Eads has suggested that the king from whom the place received its name was a man named Mormon and that he was one of the Nephite kings who reigned in the land of Nephi before the departure of Mosiah. Eads draws attention to the Nephite practice described in Alma 8:7: "Now it was the custom of the people of Nephi to call their lands, and their cities, and their villages, yea, even all their small villages, after the name of him who first possessed them . . ." [John A. Tvedtnes, "Contents of the 116 Lost Pages and the Large Plates," in The Most Correct Book, p. 47]