Hancock County, Illinois, was originally part of a military tract designated by Congress as a reward for veterans of the War of 1812. Settlement of the area was delayed due to concerns about conflicts with its indigenous peoples, but proceeded after the Blackhawk War in 1832.

The county was officially formed on January 23, 1825, out of Pike County. It was named for John Hancock, one of the signers of the Declaration of Independence. In 1833, the state commissioned the formation of the county seat at Carthage. Its original courthouse, a log cabin that later became a school, was built in Montebello (near Nauvoo, but no longer existing) and used until 1839. The second courthouse, which was in use until 1906, was the location from which Stephen A. Douglas and Abraham Lincoln gave campaign speeches while running for the U.S. Senate in 1858. The current courthouse, which was rebuilt on the same spot, was dedicated in 1908.

Hancock County is home to the city of Nauvoo, which in the 1840s was the headquarters of the Church of Jesus Christ of Latter-day Saints, and one of Illinois' most populated cities. The Church's founder, Joseph Smith, Jr., was killed at Carthage Jail in 1844, after which most Church members left the area. It continues to be a site of vacation and religious pilgrimage for Church members today.

Colonel Levi Williams was born April 8, 1794, in Madison, Kentucky, to ohn H. Williams and Elizabeth Collins. He married Mary Reid on December 27, 1814, in Madison, Kentucky, and they had five children. In the early 1830s, Levi and his family moved from Kentucky to Green Plains in Hancock County, Illinois. He was a farmer and cooper, occasionally preached as a Baptist minister, and was a county commissioner in charge of building roads. In 1835, he was given a commission as captain of the 59th regiment of the Illinois militia; five years later, in 1840, he became its colonel and commanding officer.

When Latter-day Saints began settling in Hancock County in the late 1830s and early 1840s, Williams quickly became a fierce opponent. In 1843, he led the militia that kidnapped Church member Daniel Avery and his son, threatened them with weapons, and let them go in Missouri.

After the murder of Joseph and Hyrum Smith at Carthage Jail on June 27, 1844, Levi was accused of having ordered his regiment to participate in storming the jail, but he and four others were acquitted of the charges by a jury. Williams was said to have told "Wild Bill" Hickman that the Saints held too much political influence in the county, and that killing the Smiths was the only way to get rid of them.

After the trial, Levi served as postmaster of Green Plains until his death due to a stroke on November 20, 1860.