Hosea B. Horn was born on December 3, 1820, near Harrodsburg, Kentucky, to John Horn. His father was a farmer and a veteran of the War of 1812. When he was fifteen, he became an apprentice in a printing office. He moved to Bartholomew County, Indiana, around 1839 and spent the next five years working as a clerk. He joined the Whig Party in the 1840s, and he began publishing a party newspaper in Columbus, Indiana, in 1844.
Horn moved to Davis County, Iowa, around 1845 and earned admission to the bar. He married Margaret Weaver on December 9, 1847, and they had at least two children: Clotilda, born around 1849; and Martha, born around 1858. Horn worked as a merchant, and he became postmaster on Bloomfield in April 1849. By 1850, he owned $50 of personal property. He travelled to California in 1850 and published an account of his journey. He returned to Bloomfield, Iowa, in the early 1850s, and the local Whig Party nominated him for county treasurer in 1852.
He joined the Know Nothing Party in the mid-1850s and edited a party newspaper in Bloomfield. He became a Republican around 1858. By 1860, he owned $4,500 of real estate and $3,000 of personal property. During the summer of 1862, he became a captain in the Southern Border Brigade. He also helped oversee the draft in Davis County in 1862 and served as an enrolling officer a year later.
The family moved to Atchison, Kansas, in the late 1860s, and Horn worked as a “newspaper agent” there. By 1880, he was working as an oculist. Two decades later, he had become an insurance agent. He died in Atchison on August 6, 1910.