Admitted to the bar in 1836 after teaching for a brief period, Strang also served as postmaster for five years at Ellington, N.Y., and owned and edited a weekly paper. In 1843 he followed his wife's family to Burlington, Wis. He met Joseph Smith (Smith, Joseph) the next year in Nauvoo, Ill. Despite an earlier philosophical skepticism, Strang became a Mormon convert and was ordained an elder by Smith.

After Smith's assassination in June 1844, Strang exhibited a letter, purportedly written by Smith, that named Strang his successor. He also claimed to have had a vision appointing him “seer, revelator, and prophet” of the Mormon Church. He was out-generaled, however, by Brigham Young (Young, Brigham), and the Twelve Apostles denounced Strang as an impostor and forger and expelled him from the church. Young became Smith's successor. Nevertheless, Strang persuaded a group of his own followers, including Smith's brother William, to move to Voree, Wis. (near Burlington), where they organized the new sect. There in 1845 he allegedly translated (with the aid of magic spectacles given him by an angel) The Book of the Law of the Lord from golden plates from the Ark of the Covenant. Strang then established a secret society that swore allegiance to him and operated under puritanical rules.

Dissension prompted Strang to relocate the colony in 1847 to Beaver Island, in northern Lake Michigan. By 1849 the city of St. James was established on the island's northern coast. The following year Strang received another revelation in the “plates of Laban.” It sanctioned polygyny, and he was married to four wives at one time. He also claimed that it sanctioned his coronation, and in July he became King James I. The Strangites endured considerable persecution, but Strang was able to preserve the sect and to gain acquittal in the several lawsuits brought against him.

Twice elected to the legislature in Michigan (1852, 1854), Strang had more than 5,000 followers when he was shot on June 16, 1856, by two former Strangites. Though he knew death was near, he refused to appoint a successor. Lake Michigan fishermen subsequently drove more than 2,000 Strangites from their homes and all but extinguished the sect.