Casey joined the 7th New York Militia in April 1861, just after the outbreak of the American Civil War, and was commissioned as a second lieutenant of the 5th Infantry Regiment in August of the same year. He was promoted to captain in December 1863, and became Commissary of Musters of the Army of the Potomac. He was brevetted major in March 1865 for his actions in the Battle of Fort Stedman.
He was promoted to major of the 17th Infantry Regiment in June 1884 and to lieutenant colonel of the 1st Infantry Regiment in April 1890.
He was eventually promoted to colonel of the 22nd Infantry Regiment in January 1895, and retired two years later. Casey was buried at Vale Cemetery in Schenectady, New New York