John Kirby was born on September 13, 1826, in Gloucester County, New Jersey, to John Kirby and Beulah Clark. His father was a farmer who owned $3,450 of real estate in 1850. The family lived in Gloucester County, and Kirby began working as a farmer by 1850. He married Harriet Fithian Garrison in Swedesboro, New Jersey, on March 23, 1854, and they had at least two children: Helen, born around 1855; and Catherine, born around 1856. The family moved to Mercer County, New Jersey, sometime before 1880, and Kirby worked as a physician there. He died there on February 27, 1897.