He was a Lakota war leader of the Oglala band in the 19th century. He took up arms against the United States federal government to fight against encroachment by white American settlers on Native American territory and to preserve the traditional way of life of the Lakota people.

Crazy Horse was named Čháŋ Óhaŋ (Among the Trees) at birth, meaning he was one with nature. His mother, Tȟašína Ȟlaȟlá Wiŋ (Rattling Blanket Woman), gave him the nickname Pȟehíŋ Yuȟáȟa (Curly Son) or Žiží (Light Hair) as his light, curly hair resembled her own.

His father, born in 1810, was also named Crazy Horse until 1858 when, following a battle with Arapaho warriors, Among the Trees was given the name Crazy Horse, while his father took the name Worm.

Crazy Horse lived in a Lakota camp in present-day Wyoming with his younger half-brother, Little Hawk. In 1854, Lieutenant John Lawrence Grattan and 29 other U.S. troopers entered the camp to arrest a Miniconjou man for stealing a cow. The cow had wandered into the camp, and after a short time someone butchered it and passed the meat out among the people. When the soldiers fatally shot Chief Conquering Bear, the Lakota returned fire, killing all 30 soldiers and a civilian interpreter in what was later called the Grattan massacre. After witnessing the death of Conquering Bear, Crazy Horse began to get trance visions.

During a vision quest with his father, Crazy Horse learned that his guardian spirit would be a white owl, which according to Lakota spirituality was a bestower of long life.

Crazy Horse received a black stone from a medicine man named Woptura to protect his horse, a black-and-white pinto he named Inyan, meaning "rock" or "stone". He placed the stone behind the horse's ear so that he and his horse would be one in battle. He believed the sacred stone would protect him from bullets. Subsequently, Crazy Horse was never wounded by a bullet while riding his horse.

Thanks to his fighting ability and his generosity to the tribe, Crazy Horse was named an Ogle Tanka Un ("Shirt Wearer"), making him part of a small group of distinguished community leaders who looked after the welfare of their people and made decisions about hunting grounds, campsites, and war.

In the fall of 1870, Crazy Horse invited Black Buffalo Woman to accompany him on a buffalo hunt. She was the wife of No Water, who had a reputation for drinking too much. No Water tracked down Crazy Horse and Black Buffalo Woman in the Slim Buttes area of present-day South Dakota. Finding them in a teepee, No Water stuck a pistol into the teepee and aimed for Crazy Horse. Touch the Clouds, Crazy Horse's cousin, was sitting in the teepee nearest the entry and knocked the pistol upward as No Water fired, deflecting the bullet to Crazy Horse's upper jaw. Several elders convinced Crazy Horse and No Water that no more blood should be shed. As compensation for the shooting, No Water gave Crazy Horse three horses. But because Crazy Horse was with a married woman, he was stripped of his title as Shirt Wearer.

His participation in several famous battles of the Black Hills War on the northern Great Plains, among them the Fetterman Fight in 1866 in which he acted as a decoy and the Battle of the Little Bighorn in 1876 in which he led a war party to victory, earned him great respect from both his enemies and his own people.

The battle was an overwhelming victory for the Lakota, Northern Cheyenne, and Arapaho, who were led by several major war leaders, including Crazy Horse and Chief Gall, and had been inspired by the visions of Sitting Bull. The U.S. 7th Cavalry, a force of 700 men, suffered a major defeat while commanded by Lieutenant Colonel George Armstrong Custer. Five of the 7th Cavalry's twelve companies were annihilated and Custer was killed, as were two of his brothers, a nephew, and a brother-in-law.