James B. Cahill was born on July 21, 1838, in Utica, New York, to Thomas and Margaret Cahill. His parents were Irish immigrants, and his father worked as a farmer. The family moved to Indiana in the late 1830s, and then settled in Iowa City, Iowa, in the 1840s. By 1856, he was working as a laborer. Cahill moved to Carthage, Illinois, around 1858 and worked as a deputy sheriff.
On May 4, 1861, he received a commission as a 1st lieutenant in Company D of the 16th Illinois Infantry. According to his service records, he was 5 feet, 10 inches tall, with brown hair and grey eyes. He was promoted to captain on May 14, 1861, and then to lieutenant colonel on November 23, 1862. He was devoted to the Union, and he condemned Confederates for “dishonor[ing] our flag.” In April 1862, he noted that he had “always been conservative and am yet but my experience has driven every feeling of pity and sympathy for the South from my heart.” He mustered out on December 25, 1864.
Cahill married Ellen Maude Cherill on January 29, 1862, and their daughter Maude was born around 1866. He returned to Carthage after the war and became assessor of Hancock County. By 1870, he was working as a collector for the Internal Revenue Service, and he owned $500 of personal property.
In January 1874, he travelled to Chicago, Illinois, to conduct some business. As one reporter noted, “immediately after arriving [he] complained of being unwell. He was suffering from nervousness, induced by lack of sleep, and it is supposed that he took an overdose of morphine for the purpose of obtaining rest.” He died in Chicago on January 9, 1874.