Civil War Union Brigadier General. He was born in New York City, New York, and graduated 11th in the United States Military Academy class of 1843. He then taught at his alma mater, served in California during the Mexican War, participated in the Indian campaigns, and acted as an aide to Brigadier General John E. Wool. Having risen to the rank of Captain, he was serving as adjutant general of the Department of Oregon when the Civil War began. Promoted to Lieutenant Colonel on September 28, 1861, he joined the staff of Major General George B. McClellan, serving during the Peninsula, Seven Days' and Antietam campaigns. When Major General Ambrose E. Burnside replaced McClellan, he remained at army headquarters. During the Battle of Fredericksburg, on December 13, 1862, he kept Burnside informed of the operations on the Union left. So accurate were his field dispatches that Burnside and Major General William B. Franklin, commander of the Left Grand Division, accepted them as correct assessments of the disastrous Union defeat. He was promoted to Brigadier General on November 29, 1862. His name, however, was never submitted for Senate confirmation, and his commission was revoked on January 22, 1863. On February 19, the War Department promoted him to Major and appointed him assistant adjutant general in the Regular Army. For the remainder of the war he executed special assignments for the department. In June 1863 he carried the secret orders replacing Major General Joseph Hooker with Major General George G. Meade as commander of the Army of the Potomac during the Gettysburg Campaign. In March 1864 he was promoted to Colonel and inspector general and a year later received the brevets of Brigadier and Major General, United States Army. After the war, he served as one of the army's 4 inspector generals. His military career would last for 37 years until his death on active duty, where upon he was honored as one of the most respected soldiers to serve in staff command.

He and his wife Margaret Culyer Hunter had eight children. (The name is spelled Hardy in the 1870 Census while they were living in Chicago, Illinois)
Henry Donnel Hardie, 1852-1853
Francis Hunter Hardie, 1854-1912
Margaret C. Hardie, 1855-1863
Joseph Culyer Hardie, 1857-1914
Arthur Hardie, 1860-unknown
Caroline M. Hardie, 1861-1920
Catherine Maley Hardie, 1862-unknown
Isabella "Belle" Hunter Hardie, 1865-unknown