Myles W. Keogh was born in 1840 in a small town on the river Barrow in Carlow County, Ireland. His family was farming barley, and was therefore not directly affected by the potato famine around 1845. At the age of twenty, Keogh volunteered to rush to the defence of Pius IX, since Italian nationalists made military gains against the . As a college graduate, he was appointed Officer (2nd Lieutenant) in the Battalion of St. Patrick, Papal Army at Ancona, Italy. However, the Papal forces were defeated in September 1860 during the Battle of Castelfidardo, and Keogh became a . Released after a prisoner-exchange, Keogh proceeded straight to Rome in order to become a member of the Vatican Guard. During his service, he was awarded the Pro Petri Sede Medal for gallantry and the Cross of a Knight of the Order of St. Gregory the Great.

When the broke out in 1861, the Secretary of State was actively looking for experienced European officers to serve in the . Keogh was recruited and travelled to Washington in 1862, where he was given Captain’s rank. Only days later, he confronted Confederate troops during the . He served as a staff officer during the , (also known as the single bloodiest day in American history,) and after the historic battle, was reassigned to the famous Cavalry Officer General . Keogh served Buford with obedience and gallantry during the Stoneman Raid, and the enormous all-cavalry on June 9th, 1862. Keogh rode alongside General Buford into the town of Gettysburg. He was acutely aware of the importance of holding the tactically important high ground around Gettysburg, and so he did. The Union army scored a highly significant victory. Keogh was promoted to Major for gallant and meritorious service during the battle. When General Buford succumbed to typhoid, Keogh would stay by his side and care for him, and attended his funeral at Washington. In July 1864 Major Keogh participated in raids behind Confederate lines and was promoted to Lieutenant Colonel for his gallantry during the Battle of Dallas. However, he was captured during the battle of Sunshine Church in after his horse was shot out from under him. He was released after nearly 3 months through Union General Sherman’s personal efforts.

After the end of the Civil War, Keogh was honourably discharged. Keogh, who had a handsome, athletic and dashing appearance, was also fond of the ladies but never married. He stated that “my great weakness is the love I have for the fair sex, and pretty much all my trouble comes from or can be traced to that charming source.” He accepted a commission as 2nd Lieutenant in the 4th Cavalry on May 4th, 1866, but was promoted to Captain and reassigned to the 7th Cavalry on July 28th, 1866. He would command Company I under the very controversial figure, Lieutenant Colonel George A. Custer. Keogh did have sole responsibility for defending the Smoky Hill trail against Indian raids from late 1866 to the summer of 1867. Keogh was fighting Indians almost every day. He voiced his frustration about the fact that the Native Americans did not fight in a conventional manner.

In 1874, Keogh purchased a $10,000 life insurance policy and wrote a letter to his close friends outlining his burial wishes. He gave out copies of his will to comrades, and left behind personal papers with instructions that they be burned if he was killed. Keogh died during the Battle of Little Bighorn, commonly known as Custer's Last Stand, on June 25th, 1876. The senior Captain, among Lieutenant Colonel Custer and five other companies were entirely wiped out that day. Keogh was killed in a last stand of his own, surrounded by soldiers of Company I. Keogh's body was found three days later at the centre of a group of troopers that included his two sergeants, the company trumpeter, and guidon bearer. Keogh was stripped but not mutilated, perhaps because of the Agnus Dei he wore on a chain around his neck, or the papal medals he had on him. Many Native Americans were Catholic. Keogh's badly injured horse “” was found by Sergeant Milton J. DeLacey on the fatal battlefield, in a ravine where it had crawled to die. It was raised up and tenderly cared for. Its wounds were serious but it was nursed back to health until it died in 1890. It carried seven scars from bullet wounds. Comanche is often described as the sole survivor of Custer's detachment. Keogh's bloody gauntlet and the Company’s guidon were recovered by cavalrymen three months later at the Battle of Slim Buttes.

Captain Myles Keogh is buried at Fort Hill Cemetery, New York. Additionally, a marble stone marker has been placed on the battlefield, indicating precisely where he fell.