In the Book of Mormon, the Brother of Jared is the most prominent person in the account given in the beginning (Chapters 1–6) of the Book of Ether. The Brother of Jared's name is not given in the text of the Book of Mormon, but Joseph Smith stated in 1834 that it was Mahonri Moriancumer.

In an 1835 letter, Oliver Cowdery parenthetically gives the name of the brother of Jared as "Moriancumer", which appears in the Book of Ether in the Book of Mormon only as a place name. A later account describes an 1834 statement by Joseph Smith in which he said that the brother of Jared's name had been revealed to him as "Mahonri Moriancumer":

While residing in Kirtland Elder Reynolds Cahoon had a son born to him. One day when President Joseph Smith was passing his door he called the Prophet in and asked him to bless and name the baby. Joseph did so and gave the boy the name of Mahonri Moriancumer. When he had finished the blessing he laid the child on the bed, and turning to Elder Cahoon he said, the name I have given your son is the name of the Brother of Jared; the Lord has just shown [or revealed] it to me. Elder William F. Cahoon, who was standing near, heard the Prophet make this statement to his father; and this was the first time the name of the brother of Jared was known in the Church in this dispensation.

Etymologies have been proposed but remain largely speculative.

According to Daniel H. Ludlow, it is not clear why the name of the Brother of Jared does not appear in the Book of Mormon. However, the following are possible reasons: