Jerry Ainsworth notes that at the beginning of the sixth chapter of the book of Mormon, Mormon undertakes to "finish my record concerning the destruction of my people, the Nephites" (Mormon 6:1). In other words, he has witnessed the complete or almost complete destruction of his people, so that he can now tell about it. According the Ainsworth, Mormon 6 corresponds with, and in some ways parallels, Mormon's words near the beginning of the Words of Mormon. There, he says that he has "witnessed almost all the destruction of my people, the Nephites" and he is "about to deliver up the record which I have been making into the hands of my son Moroni" (Words of Mormon 1:1; emphasis added). It appears that both of these writings--Mormon 6 and the Words of Mormon--were written about the same time.
Mormon also says that he has added the small plates of Nephi to his own abridgment or record, which record he takes from the large plates of Nephi (see Words of Mormon 1:6, 9). He again says, after witnessing "almost all the destruction" of his people, that he will now "finish out my record" of the history of the Nephites (Words of Mormon 1:9; emphasis added). His language in the two writings is thus virtually identical.
The "destruction" of the Nephites, of which Mormon speaks in Mormon 6 and the Words of Mormon, almost certainly refers to that at Cumorah.
Additionally, in Mormon 6:16, Mormon says, "My soul was rent with anguish, because of the slain of my people." Had Mormon written this immediately after the battle at Cumorah, he would probably have said, "My soul is rent."
These things mean that Mormon had time to "finish [his] record" after Cumorah, both his account of Nephite history on the large plates of Nephi and his abridgment of that history. [Jerry L. Ainsworth, The Lives and Travels of Mormon and Moroni, p. 196]