Orson Pratt died of complications from diabetes on October 3, 1881, the last surviving member of the original Council of the Twelve.

Orson Pratt was the younger brother of Parley P. Pratt, who introduced him to Latter Day Saint church and baptized him on his nineteenth birthday, September 19, 1830 in Canaan, New York.

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

Orson Pratt was a member of the original Quorum of the Twelve Apostles under Joseph Smith. He was ordained to this position on April 26, 1835. He served as a member of the mission of the Twelve Apostles to the British Isles between 1839 and 1841. He contributed to the mission by preaching in Scotland, and producing an early missionary tract, An Interesting Account of Several Remarkable Visions. This tract contains the earliest known public printing of an account of Joseph Smith's First Vision and also contains material similar to that later published as the 1842 Articles of Faith.

On his return to America in 1841, Pratt found the Church membership in contention over several issues. Rumors and gossip were rife in Nauvoo, Illinois and Pratt found the religious principle of plural marriage difficult to accept. He rebelled against Smith when he found that his wife, Sarah Pratt, accused Smith of attempting to seduce her. Pratt was disciplined and excommunicated August 20, 1842. Some months later, he reconciled with Smith and requested re-baptism. Pratt was reinstated in the Quorum of the Twelve Apostles on January 20, 1843.

After settling in the western United States with the Mormon pioneers, Pratt was called to return to Europe as a mission administrator between 1848 and 1851. In 1850, as mission president of the British Mission, Pratt told his missionaries "[e]very soul in Britain should hear the gospel this year".[citation needed] Although this goal was not achieved, by the end of the year there were twice as many church members residing in Britain as in the United States.

While presiding over this mission, Pratt received a pamphlet from Lorenzo Snow entitled "The voice of Joseph" that Snow wanted translated into French to advance his missionary efforts in northern Italy. Pratt managed to make contacts with people in Paris who were willing to do this translation.

In 1865, Pratt was one of the first LDS missionaries to work in Austria. Traveling with William W. Ritter, he was there for nine months, but did not baptize anyone. The LDS missionaries were eventually expelled by the Austrian government.

Pratt was a member of Young's initial pioneer company, the "Vanguard Company," that crossed the plains to select a western site for Mormon colonization. His journals of this trip are an important Mormon history resource. As the group made their way from Missouri to Utah, Pratt acted as the company's scientific observer. He made regular readings with the company's scientific instruments, took notes on geological formations and mineral resources, and recorded information on plants and animals. He described snow on Laramie Peak on June 7, and noted that rock found on June 10, ...would make excellent grindstones, being of fine grit sandstone. (May, p. 57)

As a mathematician, Pratt assisted company scribe William Clayton in the design and invention of a version of the modern odometer. Intended to compute the distance traveled per day, the design consisted of a set of wooden cog wheels attached to the hub of a wagon wheel, with the mechanism "counting" the revolutions of the wheel. The apparatus, called the "roadometer," was built by carpenter Appleton Milo Harmon, and was first used on the morning of May 12, 1847.