John Cassels was born on July 29, 1835, in Montreal, Canada, to James and Margaret Cassels. He was baptized on September 13, 1835. His family immigrated to America around 1840, and he probably settled in Pittsburgh, Pennsylvania.
He enlisted in the Union army, and he mustered in as a private in Company B of the 12th Pennsylvania Infantry on April 25, 1861. He mustered out on August 5, 1861. In November 1861, he received a commission as a 1st lieutenant in Company C of the 11th Pennsylvania Cavalry. He was promoted to captain in April 1862. In December 1863, army officials assigned him to serve as provost marshal at Fortress Monroe. He was promoted to major in October 1864, and he resigned on April 1, 1865.
He apparently joined the Regular Army after the war. He married Annie Sewall on November 11, 1868, and their son John was born around 1871. By 1870, he was serving in Wauwatosa, Wisconsin, and he owned $4,000 of personal property. His wife died on June 14, 1871, and he married Ellen W. Fletcher on January 14, 1874. They had at least three children: Arthur, born around 1874; Margaret, born around 1875; and Elsie, born around 1879. He apparently resigned from the army in the 1870s, and he settled in Washington, D.C. He earned a living as a lawyer and, later, as a railroad official. He applied for a federal pension in October 1905 and eventually received one. He died of a cerebral hemorrhage in Washington, D.C., on March 28, 1908.