[2] He initially attended Williams College, where he was a member of the Kappa Alpha Society, and then accepted a nomination to the United States Military Academy, where he graduated at the head of his class in 1862.

Commissioned a second lieutenant in the Corps of Engineers, Mackenzie served in the battles of Second Bull Run, Antietam, Gettysburg, and through the Overland Campaign and Petersburg in 1864.

Upon his recovery, on November 30, 1864, President Abraham Lincoln appointed Mackenzie brigadier general of volunteers, to rank from October 19, 1864.

[3] After his appointment, Mackenzie assumed command of the Cavalry Division in the Army of the James, which he led at the battles of Five Forks and Appomattox Courthouse.

He wrote, "Griffin, Humphreys, and Mackenzie were good corps commanders, but came into that position so near to the close of the war as not to attract public attention.

Mackenzie fought in the Red River War, routing a combined Indian force at the Battle of Palo Duro Canyon far to the north from his headquarters at Fort Concho in San Angelo, Texas.

In 1876, he defeated the Cheyenne in the Dull Knife Fight, which helped bring about the end of the Black Hills War.

He bought a Texas ranch and was engaged to be married; however, he began to demonstrate odd behavior which was attributed to a fall from a wagon at Fort Sill, Oklahoma, in which he injured his head.

[5] Mackenzie died at his sister's home in New Brighton, Staten Island, New York, and is buried in West Point National Cemetery.

However, the Army and Navy Journal carried a lengthy article on his career and personal life, which began, "The sorrow with which the Army will learn of the death of the once brilliant Ranald Slidell MacKenzie derives an additional pang from the recollection of the cloud which overshadowed his later years and consigned him to a living death."