About 310 AD: Mormon is born probably somewhere in the land northward, which usually is not part of the Book of Mormon narrative. We know little of his background other than his father's name also being Mormon
About 320 AD: The last record keeper, Ammoron, apparently not a relative of Mormon, gives the 10-year old boy an important message: When he is about 24, he needs to go to the hill Shim where all the Nephite records are hidden, retrieve the plates of Nephi and make a record.
About 345 AD: Mormon retrieves the plates of Nephi in the hill when his armies are close to the site. He is about 35 years old, 11 years older than he was supposed to be when getting the records. Warfare has probably prevented him until this time. He makes a record on the plates, even "a full account" (Mormon 2:17-18). Note: This is not the account we have in the Book of Mormon today
About 384-385 AD: Mormon is now about 74 years old. It is just before the final battle between the Nephites and Lamanites. Mormon creates a new record (plates of Mormon) on which he engraves an abridgment of the plates of Nephi before he makes his own account and then hands the plates to his son, Moroni. His own account on the "few" plates (Mormon 6:6) he made seems to be a shorter version than the one on the plates of Nephi (Mormon 2:18).
What seems to also have happened in the meantime, between 345 and 384, is that he retrieved all of the Nephite records from the hill, Shim. In 345 he only took the plates of Nephi and left the rest there. But in 384, he says that he had
been commanded of the Lord that I should not suffer the records which had been handed down by our fathers, which were sacred, to fall into the hands of the Lamanites, (for the Lamanites would destroy them) therefore I made this record out of the plates of Nephi, and hid up in the hill Cumorah all the records which had been entrusted to me by the hand of the Lord, save it were these few plates which I gave unto my son Moroni.Since Lamanites have captured all the lands south of Cumorah, Mormon must have taken the records out of Shim before the Lamanites possessed that part of the land. Now everything, except the plates of Mormon is hidden in Cumorah instead.
When Mormon has abridged about half of the Nephite record, he discovers the small plates of Nephi (see Words of Mormon 1:3). Then he attaches them to the plates of Mormon (somehow?). This could indicate that he hadn't had the plates for very long or at least hadn't had the time in the middle of warfare to look through all of them, or perhaps there really just was a very large amount of records. As they now spent years gathering the people for the last battle, he had some time on his hands and it seems to be the time period where he is making this abridgment. At this time, Mormon has already lived a long life and "witnessed almost all the destruction of [his] people" (Words of Mormon 1:1).
400 AD: The next thing we know is Moroni finishing his father's record, telling us that 400 years have passed and Mormon has died sometime after the last battle at Cumorah We don't know exactly when, but at least we know he died sometime between age 75 and 90.
A few notes:
- Mormon was only 10 years old when he received instructions about the Nephite records, but the Book of Mormon (in the Book of Mormon) was probably written when he was more than 70 years old.
- Mormon made two accounts: One on the plates of Nephi, which we don't have, one on the plates he made himself which contain the abridgment from the plates of Nephi. Like Nephi himself, he first made a larger account, then a smaller account, and we only have the smaller.
- It sounds like Mormon has a lot of records in addition to the plates of Nephi and the plates he made. We learn from Helaman 3:15 that "there are many books and many records of every kind, and they have been kept chiefly by the Nephites." Helaman 3 talks about the people "who were in the land northward" (verse 9), so it seems that the many books and records are kept by the people there. It is worth noting that the hill, Shim, where the records were hidden, is in the land northward.