Joseph J. Comstock Jr. was born around 1838 in Providence, Rhode Island, to Joseph and Maria Comstock. His father was a sea captain. The family lived in Jersey City, New Jersey. He travelled abroad in 1860, and his passport described him as 5 feet, 9 inches tall, with dark brown hair and hazel eyes.
He joined the 7th New York Militia Infantry in 1861. That fall, he received a commission as a 1st lieutenant in the 3rd Rhode Island Heavy Artillery. He was promoted to captain in March 1862. Then, in late 1863, he was promoted to major in the 14th Rhode Island Colored Heavy Artillery, which eventually became the 11th USCT Heavy Artillery. The regiment served in the Department of the Gulf, and he mustered out on July 25, 1865.
He settled in Providence after the war, and he earned a living as a merchant. By 1870, he owned $2,100 of real estate. He married a woman named Anna around 1878, and the couple had no children. He applied for a federal pension in June 1880, but he apparently never received one. By 1900, they were living in New York City, New York, and he was working as a publisher. He died of pneumonia in New York City on March 19, 1903.