The Book of Mormon recounts the history of a lost tribe of Israel who were led away from Israel in about 600 BC to America. Upon arriving in America, the families of Lehi and Ishmael separated into 2 groups known afterward as the Nephites amd Lamanites. The Book of Mormon recounts the eventual conversion of many of the Lamanites to the Lord and at the same time the increasing pride and apostasy of the, once faithful, Nephites because of their prosperity.
At a certain point just prior to the appearance of Jesus Christ in America, a Lamanite prophet was raised up among the Lamanites who gave a powerful but brief sermon and witness of Jesus Christ to the more wicked Nephites.
I don't know if Samuel the Lamanite held amy official position in the Ancient American Church, but one important point of his preaching was that Samuel did not draw people away from the Lord's prophet but led people to the prophet Nephi who was at the head of their Church to repent and be baptized.
President Joseph Smith, Jun., addressed the assembly and said, "the Melchizedek High Priesthood was no other than the Priesthood of the Son of God; that there are certain ordinances which belong to the Priesthood from which flow certain results; and the Presidents or Presidency are over the Church; and revelations of the mind and will of God to the Church are to come through the Presidency. This is the order of heaven." - History of the Church, Volume Two, page 477