He also served as a representative to the Utah Territorial Legislature and is most known for writing a public confession to committing several murders under orders from Church of Jesus Christ of Latter Day Saints prophet Brigham Young.

During this period Hickman began his practice of polygamy under the instruction of Brigham Young, marrying Sarah Elizabeth Luce as his second wife in January 1846.

During the course of violence, Hickman records killing the chief of the band, Old Elk, whose head he removed with his Bowie Knife and which he brought back to Fort Utah and hung from the walls.

Hickman was sent along with a posse of 150 Mormon militia who killed a couple of the mountain men and stole several hundred head of stock and whiskey they found at the fort, although Jim Bridger himself was not there.

[2]: 93 In April 1854, fearing retribution by the mountain men or allied Native bands, Hickman was asked by Young to go to Green River and establish a ferry under church ownership.

Instead, Hickman established a prosperous trading post at Pacific Springs near South Pass, 26 miles (42 km) east of Green River.

[4]: 53 On February 8, 1856, Hickman, along with Porter Rockwell, and at the request of Brigham Young, carried the mail from Independence, Missouri, to Salt Lake City.

Hickman, according to his autobiography, first began to break with Brigham Young during the latter's handling of the Morrisite War in 1863, claiming to put his name alongside those of many Utah non-Mormons to an ultimately successful petition to have the surviving prisoners of the minor sect released by a pardon from the (non-Brighamite) territorial governor.

[2]: 183  In 1868 while Hickman was still in California his Taylorsville, Utah Bishop excommunicated him from the LDS Church, and with his plural marriages no longer being in force, ultimately nine of his ten wives would leave him.

[2]: 192–193  While at Fort Douglas, he wrote his autobiography, which was later given to J. H. Beadle who performed some basic fact-checking and ultimately published it under the title "Brigham's Destroying Angel".

Hickman, who had struck a deal with federal law enforcement to testify against Young if he were ever to be brought to trial, was never convicted of the crimes to which he confessed.