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As the surveyor for the Newton-Jenney Party, McGillycuddy was the first known person to climb Harney Peak in the Black Hills of South Dakota. He then served as Contract Surgeon with General George Crook during the Battle of the Rosebud, the Battle of Slim Buttes, and the Horsemeat March.
As Assistant Post Surgeon at Fort Robinson and later as Indian Agent for the Red Cloud Agency, McGillycuddy was known to the Lakota of the modern-day Pine Ridge Indian Reservation as "Friend of Crazy Horse", and was the doctor who treated Crazy Horse at the time of his death.
While he may have been a friend to Crazy Horse, he was not so much loved by some other Lakotas, including Red Cloud, a major Sioux chief. Red Cloud's accusations of mismanagement led to several investigations of Dr. McGillycuddy's administration. Under pressure to arbitrarily fire a loyal clerk, Dr. McGillycuddy eventually resigned his post. Later, in the days leading up to the Wounded Knee Massacre, Red Cloud conceded that McGillycuddy had been a "young man with an old man's head on his shoulders."
Dr. McGillycuddy would later serve as mayor of Rapid City, Dean of the South Dakota School of Mines and Technology, delegate to the South Dakota State Constitutional Convention, and as South Dakota's first State Surgeon General. The mansion that he built in 1888 is still standing today in Rapid City.
McGillycuddy died at age 90 in Berkeley, California. His ashes are entombed at the top of Harney Peak behind a plaque that reads "Valentine McGillycuddy, Wasitu Wacan (Lakota: translates as Holy White Man)."
Books based on Valentine McGillycuddy's life include:
- McGillycuddy, Agent (Julia McGillycuddy, 1942); later republished as Blood on the Moon
- Contract Surgeon (Dan Obrien, 2001)
- Indian Agent (Dan Obrien, 2005)
- Heroes Without Glory: Some Good Men of the Old West (Jack Schaefer, 1965)
 Museum and archival collections
- Journey Museum, Rapid City, South Dakota
- State Historical Archives, Pierre, South Dakota
- University of South Dakota, Vermillion, South Dakota
- McGillycuddy House, Rapid City, South Dakota
- South Dakota School of Mines and Technology, Rapid City, South Dakota
- Fort Robinson Historical Museum, Fort Robinson, Nebraska
- National Anthropological Archives, Smithsonian Institution, Suitland, Maryland
- The Killing of Chief Crazy Horse: Three Eyewitness Views by the Indian Chief He Dog, the Indian White, William Garnett, the White Doctor, Valentine McGillycuddy
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