Amaleki was born "in the days of Mosiah" (Omni 1:23), which probably means that he was born during the reign of Mosiah1. [See the commentary on Omni 1:12]
Omni 1:23 I [Amaleki] have lived to see [Benjamin's] death ([Illustration]): Book of Mormon Structure. The span of years from Jacob to the last days of Amaleki amount to roughly 400 years; which means that in just a few pages the reader has covered nearly 40 percent of the time frame for the history of the people of Lehi. With this in mind, we should be wary of calling the Book of Mormon a "history of the people who lived on the American Continent before Columbus." [Gerald Lund - personal file]
Omni 1:23 And Benjamin, his [Mosiah's son] reigneth in his stead (Major Nephite Leaders) [[Illustration]]: The Major Leaders During Nephite History. [Church Educational System, Book of Mormon Student Manual: Religion 121 and 122, 1989, p. 160]