John H. Horn was born on October 5, 1840, in Salem, Virginia, to William Horn. His father was a farmer who owned $325 of personal property by 1860. The family moved to Jacksonville, Illinois, in the early 1850s, and by 1860, Horn was working as a farm laborer. He married Mary Rynearson on August 20, 1862, and they had four children: Medora, born around 1863; William, born around 1866; Susan, born around 1867; and Bertha, born around 1874.
He enlisted in the Union army on August 7, 1862, and he mustered in as a private in Company B of the 101st Illinois Infantry on September 2, 1862. According to his service records, he was 5 feet, 7¾ inches tall, with black hair and grey eyes. Army officials transferred him to the Mississippi Marine Brigade on February 15, 1863. He remained in the military until at least June 1864.
Horn returned to Jacksonville and resumed his work as a farmer. He remained in Jacksonville for the rest of his life, and he died there on September 9, 1928.