Josiah P. Chadbourne was born around 1828 in Baldwin, Maine, to Joshua and Charlotte Chadbourne. His father was a merchant who owned $2,000 of real estate by 1850. He grew up in Baldwin, and by the late 1840s, he was working as a farmer.
He enlisted in the United States Army on August 18, 1846, and he served as a 2nd lieutenant in the 9th United States Infantry. According to his service records, he was 5 feet, 10 inches tall, with brown hair and blue eyes. He took part in the Mexican American War, and he mustered out on March 31, 1847.
He married a woman named Caroline, and they had at least five children: Henry, born around 1853; Fanny, born around 1857; George, born around 1859; Joseph, born around 1861; and Nettie, born around 1869. He earned a living as a farmer, and the 1860 census described him as a “gentleman.” By 1860, he owned $1,000 of real estate and $50 of personal property. In February 1859, he received an appointment as a local postmaster. He died in Baldwin on July 9, 1890.