According to the Book of Mormon, Lehi was a son of Helaman and was a Nephite missionary. He and his elder brother Nephi lived in the 1st century BC and had many missionary experiences together. The main events of their missions are recorded in the fifth chapter of The Book of Helaman. Lehi was named after his ancestor, Lehi, whose family immigrated to the New World from Jerusalem around 600 BC. Helaman taught his two sons to keep the commandments and to walk uprightly before God, as their namesakes had done.
Lehi's brother Nephi was the Chief Judge of the Nephites. After he resigned from the position, he joined Lehi in devoting the balance of their lives to the preaching of the Word of God. Their first missionary efforts were among the Nephites, where they found success in prophesies and calling them to repentance, as they traveled to various Nephite cities.
The next phase of their labors took them to the Lamanites in the land of Zarahemla, where they also met Nephite dissenters who had joined the Lamanites. As they taught with great power, many dissenters were confounded, humbled themselves, and eventually were baptized unto repentance. These Nephites returned to their people to try to repair the wrongs which they had done.
Lehi and Nephi then turned their attention to the Lamanites where the power and authority of their words came with great astonishment to the Lamanites, insomuch that 8,000 souls were convinced of the wickedness of the traditions of their fathers, and were baptized unto repentance. Lehi and Nephi then traveled to the land of Nephi where a Lamanite army captured and cast them into prison – the same prison that once held Ammon and his brethren.