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.

Mission president in Britain

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". 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.

Opening the Austrian mission

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.

With Erastus Snow, Pratt entered the Salt Lake Valley on July 21, 1847, three days ahead of the main body of the Vanguard company. Several days later, he preached the first sermon in the Salt Lake Valley and formally dedicated the valley to the Lord.

Orson Pratt was married to ten women. At age 57 Pratt married a sixteen year old girl, his tenth wife Margaret Graham, younger than his daughter Celestia, causing his first wife Sarah, an outspoken critic of polygamy, to lash out in an 1877 interview, Pratt and all of his wives and children struggled with poverty.

1842 polygamy scandal and relationship with Sarah Pratt

In 1886, Orson's wife Sarah Pratt claimed in an interview that, while in Nauvoo, Illinois, Joseph Smith, Jr. was attracted to her and intended to make her "one of his spiritual wives" while Orson was in England on missionary service. To Smith's proposal Sarah replied, "Am I called upon to break the marriage covenant to my lawful husband! I never will. I care not for the blessings of Jacob, and I believe in NO SUCH revelations, neither will I consent under any circumstances. I have one good husband, and that is enough for me." Pratt issued an ultimatum to Smith: "Joseph, if you ever attempt any thing of the kind with me again, I will tell Mr. Pratt on his return home. Depend upon it, I will certainly do it," a warning that elicited the threat from Smith, "Sister Pratt, I hope you will not expose me; if I am to suffer, all suffer; so do not expose me. If you should tell, I will ruin your reputation, remember that." After her husband Orson returned from England, Pratt later claimed an incident between Pratt and Smith at her home occurred, and "Sarah ordered the Prophet out of the house, and the Prophet used obscene language to her [declaring that he had found Bennett in bed with her]," according to Sarah Pratt's neighbor, Mary Ettie V. Smith. Sarah told her husband about the incident; Orson took Sarah's side and confronted Smith, who denied Sarah's allegation and responded that she was Bennett's lover. The resulting estrangement between Smith and Orson Pratt, who stood by Sarah in preference to the denials of Joseph, who warned his disciple that "if [Orson] did believe his wife and follow her suggestions he would go to hell,.

However, in the local and Mormon press, Sarah Pratt was accused of having had an adulterous relationship, not with Smith, but with Bennett, and numerous affidavits were printed in local and pro-Mormon Nauvoo publications, including the leading councils of the church and from members such as Jacob B. Backenstos, a relative of the sheriff of Hancock County. Van Wagoner has dismissed the adultery charges against Sarah Pratt as "highly improbable" and that J. B. Backenstos's affidavit stating that Bennett continued the adulterous relationship with Sarah Pratt after Orson returned from England could "be dismissed as slander."

Orson Pratt became estranged from the church and Smith. Wilford Woodruff stated that "Dr. John Cook Bennett was the ruin of Orson Pratt". Van Wagoner and Walker note that, on August 20, 1842, "after four days of fruitless efforts at reconciliation, the Twelve excommunicated Pratt for 'insubordination' and Sarah for 'adultery'".

Orson soon returned to the church and denounced Bennett. Van Wagoner cites a letter written by Orson's brother Parley P. Pratt, Orson wrote a postscript to his brother's letter: "J.C. Bennett has published lies concerning myself & family & the people with which I am connected".

List of wives and children

• Sarah Marinda Bates
• Orson Pratt, Jr.
• Declined missionary service with Brigham Young because "I informed you of the change that had taken place in my religious views."
• Declared to Mormon church officials, "I was made a High Counciler, although I was then an unbeliever, as now In regard to my faith I resolved I would not accept nothing that my conscience would not receive. I have come to the conclusion that Joseph Smith was not especially sent by the Lord to establish this work, and I cannot help it, for I could not believe otherwise, even if I knew I was to suffer for it the next moment."
• Excommunicated, 18 September 1863
• Lydia Pratt
• Celestia Larissa Pratt
• Sarah Marinda Pratt
• Vanson Pratt
• Laron Pratt
• Marlon Pratt
• Marintha Althera Pratt
• Harmel Pratt
• Arthur Pratt
• Deputy U.S. marshall
• Replied to a reported in 1882 why he was not a Mormon, "I am the son of my father's first wife, and had a mother who taught me the evils of the system."
• Excommunicated for apostasy on 5 October 1874
• Herma Ethna Pratt
• Liola Menella Pratt
• Charlatte Bishop
• Adelia Ann Bishop
• Lucy Adelia Bishop Pratt
• Elzina Bishop Pratt
• Lorum Bishop Pratt
• Lorus Bishop Pratt
• Eltha Bishop Pratt
• Orthena Bishop Pratt
• Mary Ann Merrill
• Milando Merrill Pratt
• Vianna Merrill Pratt
• Oradine Merrill Pratt
• Lathilla Merrill Pratt
• Valton Merrill Pratt
• Louisa Chandler
• Marian Ross
• Marian Agnes Ross Pratt
• Larinda Marissa Ross Pratt
• Milson Ross Pratt
• Irintha Pratt
• Ray Ross Pratt
• Ruby Ross Pratt
• Sarah Louisa Lewis
• Willow Lewis Pratt
• Juliaet Ann Phelps
• Alva Phelps Pratt
• Clomenia Phelps Pratt
• Ortherus Phelps Pratt
• Margaret Phelps Pratt
• Rella Phelps Pratt
• Neva Phelps Pratt
• Julius Phelps Pratt
• Eliza Crooks
• Levius Crooks Pratt
• Dora Crooks Pratt
• Jarad Crooks Pratt
• Onthew Crooks Pratt
• Samuel Crooks Pratt
• Margaret Graham
• Orlon Graham Pratt
• Pearl Graham Pratt
• Royal Graham Pratt

While in Illinois, Pratt acted as an instructor at the university in Nauvoo.

In Utah, Pratt's strong skills in analysis and writing led Young to assign him to produce sermons and pamphlets dealing with LDS topics. He wrote sixteen pamphlets in defense of LDS doctrines, drawing on the works of Joseph Smith, Jr. and his brother Parley P. Pratt. These include Divine Authority, or the Question, Was Joseph Smith Sent of God? in 1848 and Divine Authenticity of the Book of Mormon in 1850 and 1851. Although these materials were primarily used in the mission field, Pratt was also a church spokesman on the topic of plural marriage. At a special missionary conference in Salt Lake City in August 1852, Pratt publicly preached a sermon announcing the doctrine of plural marriage. He later published an essay in defense of the practice in 12 monthly installments in the church periodical The Seer, which provides the most complete defense of the LDS doctrine during this period. Orson Pratt as part of his system of Mormon theology embraced the philosophical doctrine of hylozoism. Pratt himself practiced plural marriage, having seven wives and forty-five children.

However, Pratt's views were not always without controversy. In 1865, a majority of the First Presidency and the Quorum of the Twelve Apostles of the LDS Church officially condemned some of Pratt's doctrinal writings, including some of his articles from The Seer:

"The Seer [and other writings by Pratt] contain doctrines which we cannot sanction, and which we have felt impressed to disown, so that the Saints who now live, and who may live hereafter, may not be misled by our silence, or be left to misinterpret it. Where these objectionable works, or parts of works, are bound in volumes, or otherwise, they should be cut out and destroyed."

Pratt acted as Church Historian and Recorder from 1874 until his death. He edited many church periodicals and helped divide editions of the Book of Mormon and Doctrine and Covenants into verse format and provided appropriate cross references.

Science publications

Pratt was known as an accomplished mathematician, and had a strong interest in astronomy. He offered science based lectures on these topics to early LDS audiences in Utah, and published two related books. New and Easy Method of Solution of the Cubic and Biquadratic Equations was published in 1866, and Key to the Universe was published in 1879.