Orson Pratt was born on 19 September 1811 in Hartford, New York. In September 1830 his older brother Parley told him of Joseph Smith and the Book of Mormon. Orson converted to Mormonism, and the two brothers traveled to New York to meet Smith. Pratt remained with Smith for a month and was then called on a proselyting mission. Three years later he was recalled to church headquarters in Kirtland, Ohio, and in 1835 he was appointed to the Quorum of Twelve Apostles. Two years later he married Sarah M. Bates.

Kirtland's fragile economy collapsed in 1837. Many Mormons, including Orson Pratt, turned against Smith. But Pratt did not remain long at odds with his leader, leaving soon for a mission to New York. He reached St. Louis, where he waited out the winter after Sarah had given birth.

Following another mission, this time to England, Pratt arrived in Nauvoo, Illinois, in July 1841. During Pratt's absence, Joseph Smith had begun practicing plural marriage. One of the women he approached was Sarah, who rejected his advances. The next spring Sarah told Orson of Smith's proposals. Pratt became depressed and on 14 July disappeared; however, he was found near the Mississippi River and was persuaded to return home. In mid-August three apostles tried to convince him to sustain Smith. When he refused, they excommunicated him from the church.