Ammon began as an unbeliever and sought with his friend Alma and the other sons of Mosiah to destroy the Church of God. An angel appeared to the young men, shaking them in their sins, and all four were converted. They spent the rest of their days serving God, after courageously taking the gospel to a decadent and violent people. Ammon refused the crown of his father in order to pursue missionary work. Though he started out on his mission with his brethren, he separated from them and found his way into the land of Nephi. There he wandered into the Lamanite territory governed by King Lamoni. Ammon went with Lamoni's servants to watch over the king's flocks, and slew other Lamanites who tried to steal Lamoni's sheep. The strength Ammon manifested was super-human. That, along with the fact that he was part of an enemy nation but seemed to swear perfect loyalty to the Lamanite king, impressed the king. King Lamoni questioned Ammon, and Ammon gave credit to God. Ammon explained the gospel to Lamoni, beginning with the creation story, and eventually Lamoni was converted with all his people. Lamoni's people eventually joined with the Nephites, who received them gladly, and they remained ever-faithful in their testimonies of Jesus Christ.
Ammon continued to do the work of the Lord. The people of Lamoni later called themselves The People of Ammon or the Ammonites, and Ammon became their high priest. Ammon banished Korihor, an anti-Christ, and then served another proselyting mission to the Zoramites.