Question: Why does the book of Mosiah begin with the final days of King Benjamin?
Answer: Royal Skousen wrote, “Along with the loss of the first 116 pages of the original manuscript, which contained the book of Lehi, most of the original first two chapters of the book of Mosiah were also apparently lost.
“In the printer’s manuscript, the beginning of Mosiah was originally designated as chapter III. In addition, the title of the book (‘the book of Mosiah’) was later inserted between the lines.”
Skousen adds, “All other books [within the Book of Mormon] start their account with the person for which the book is named, yet here the book of Mosiah begins with king Benjamin.
“The original book undoubtedly began with the account of a Mosiah–Benjamin’s father, the first Mosiah. Further, this book is missing the initial book summary that typically begins all the other longer books.”
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 134