Upon the death of the President of The Church of Jesus Christ of Latter-day Saints, the senior apostle in the Church's governing quorums (see First Presidency; Quorum of the Twelve Apostles) becomes presiding officer of the Church (see President of the Church).
The principles underlying the succession process were established at the death of the Prophet Joseph Smith in 1844. Since there was at the time no precedent and no clear procedure providing for succession to the office of president, competing views arose. Brigham Young, then President of the Quorum of the Twelve Apostles, presented the proposition that the Twelve, ordained apostles who held all the keys necessary to govern the Church, should be sustained as the authorized leaders in the absence of Joseph Smith. In his favor was the fact that the Twelve in Nauvoo had been carefully tutored by the Prophet in all aspects of Church leadership and had served as his right hand. The Church also understood that this position was in harmony with the 1835 revelation on priesthood (D&C 107). After describing the First Presidency ("three Presiding High Priests, chosen by the body, appointed and ordained to that office, and upheld by the confidence, faith, and prayer of the church"), that revelation affirmed that the Twelve Apostles "form a quorum, equal in authority and power to the three presidents previously mentioned" (D&C 107:22-24).
Inherent in the Twelve's proposal was the assumption that, although the Quorum of the Twelve Apostles had equal authority and power with the Quorum of the First Presidency, as long as the First Presidency was intact and functioning, they, and not the Twelve, possess the necessary jurisdiction to govern the Church. But the death of the president, thereby disorganizing the presidency and automatically releasing the president's counselors, bestows on the Quorum of the Twelve the required authorization to exercise the keys they already possess and assume full responsibility for governing the Church-including the reorganization of the First Presidency. Representing the Twelve, Brigham Young also reminded the Saints in 1844 of Joseph Smith's "last charge to the Twelve," stipulating that in the event something happened to him, the Twelve were responsible for carrying on the work he had begun (Esplin, pp. 319-20)
Doctrine and Covenants and Church History Seminary Teacher Resource Manual, (2001), 236–38