Orson Pratt, Sr. (September 19, 1811 – October 3, 1881) was a mathematician and an original member of the Quorum of Twelve Apostles of the Church of the Latter Day Saints. He became a member of the Quorum of the Twelve of The Church of Jesus Christ of Latter-day Saints (LDS Church) and was a leading Mormon theologian and writer until his death.

Church membership and service

Pratt was born in Hartford, New York, the son of Jared Pratt and Charity Dickenson. He was the younger brother of Parley P. Pratt, who introduced him to Latter Day Saint church and baptized him on Orson's nineteenth birthday, September 19, 1830, in Canaan, New York.

Pratt was ordained an Elder several months later, on April 26, 1831, by Joseph Smith and immediately set out for Colesville, New York, his first mission. This was the first of a number of short missions in which Pratt visited New York, Ohio, Missouri, and the Eastern States. On February 2, 1832, he was ordained a High Priest by Sidney Rigdon, whereafter he continued his missions, preaching in Pennsylvania, New York, New Jersey, Vermont, New Hampshire, Connecticut, and Massachusetts.