Cooke was born in Mount Pleasant,
Brant County, Ontario, to Alexander and Angeline Cooke. He attended the Brantford Collegiate School and the Central School in
Hamilton, Ontario. At the age of 14, he moved to
Buffalo, New York to continue his studies. After graduating from school, he enlisted in 1863 with the 24th New York Cavalry at
Niagara Falls, New York, during the Civil War. After serving as a recruiting officer, he served on the front lines in the
IX Corps, commanded by
Ambrose Burnside. He was wounded during the
Siege of Petersburg. After being released from the hospital, he served on commissary duty. He rose to
first lieutenant on December 14, 1864, but did not return to front-line duty until March 1865. He was awarded
brevet promotions to
captain,
major, and
lieutenant colonel for his meritorious service during the war, the last of his actions occurred at the
Battle of Sayler's Creek during the
Appomattox Campaign. After the war, he immediately joined the 1st New York Provisional Cavalry and applied for a
Regular Army commission. He was made a
second lieutenant in the
U.S. 7th Cavalry Regiment in July 1866, and was promoted to
first lieutenant a year later at
Fort Harker in Kansas. In 1868, he participated in the
Washita Campaign. In 1871, he became the regimental
adjutant under Lt. Col. George Armstrong Custer. Cooke became close friends with
Thomas Custer and was a member of the so-called "Custer Clan" or "Custer Gang", a close-knit group of Custer's friends and relatives. He was an excellent shot and one of the fastest runners of the regiment. Some of his troopers took a dislike to him and called him "The Queen's Own". He was known for his
dundrearies, or long side whiskers, that he always wore. ==Little Big Horn==