Himni was one of the four sons of Mosiah², the Nephite king who reigned in Zarahemla; his brothers were Ammon, Aaron, and Omner (Mosiah 27:34). Before his conversion he was numbered among the unbelievers and went about with his brothers seeking to destroy the church of God and lead the people astray (Mosiah 27:8-10).
After an angel appeared to them, Himni and his brothers reversed course. They traveled throughout the land of Zarahemla to repair the injuries they had done to the church, confessing their sins and preaching the word of God (Mosiah 27:32-37). They then refused the kingdom their father offered and asked instead to go up to the land of Nephi to preach to the Lamanites (Mosiah 28:1-9; Alma 17:6).
Himni had searched the scriptures diligently and given himself to prayer and fasting, and so taught with the spirit of prophecy and revelation (Alma 17:2-3). He and his brothers preached among the Lamanites for fourteen years (Alma 17:4), with many converted (Alma 25:17; Alma 26:29-30). When Alma the Younger led a mission to the Zoramites, he left Himni in charge of the church in Zarahemla while taking the other three brothers with him (Alma 31:6).