Leonard Peltier was born on September 12, 1944, at the Turtle Mountain Indian Reservation of the Turtle Mountain Chippewa near Belcourt, North Dakota. He is of Lakota, Dakota, and Anishinaabe descent. Leonard was raised among the Turtle Mountain Chippewa and Fort Totten Sioux Nations of North Dakota. He was one of 13 children in his family. His parents divorced when he was four years old. After the divorce, Leonard and his sister Betty Ann lived with their paternal grandparents, Alex and Mary Dubois-Peltier, on the Turtle Mountain Indian Reservation.

In September 1953, Peltier was sent to the Wahpeton Indian School in Wahpeton, North Dakota. This Indian boarding school was run by the Bureau of Indian Affairs. It was located 150 miles away from his home. The school practiced forced cultural assimilation, requiring the children to use English and forbidding Native American culture. Leonard graduated from Wahpeton in May 1957. After that, he attended the Flandreau Indian School in Flandreau, South Dakota. He completed the ninth grade there and then returned to the Turtle Mountain Reservation to live with his father. Leonard later obtained a general equivalency degree (GED).

In 1965, Leonard moved to Seattle, Washington, where he worked as a welder and a construction worker. He also became the co-owner of an auto shop in Seattle. The shop's upper level was used as a halfway house for American Indians who were recovering from alcohol addiction or who had recently finished their prison sentences. Peltier became involved in various causes for Native American civil rights while living in Seattle.

By the early 1970s, Peltier learned about tensions at the Pine Ridge Indian Reservation in South Dakota, where issues arose between supporters of Richard Wilson, the elected tribal chairman, and traditionalist members of the Lakota tribe. In 1972, Leonard became an official member of the American Indian Movement (AIM), a group working to advocate for Native American civil rights.

During his time with AIM, Peltier had seven children from two marriages and also adopted two children.

On June 26, 1975, a critical shootout occurred on the Pine Ridge Indian Reservation. FBI Special Agents Ronald Arthur Williams and Jack Ross Coler were pursuing a suspect named Jimmy Eagle when they encountered a vehicle driven by Leonard Peltier, along with two other men. They followed this vehicle, described interchangeably as a red pickup truck or an orange and white Chevrolet Suburban.

As the agents approached, the occupants of the vehicle exited and a firefight began. The agents communicated that they were under fire and needed assistance. Unfortunately, both agents were shot and died within minutes of the gunfire starting. A total of 125 bullet holes were found in their vehicles, with evidence indicating that close-range shots to the heads of the agents contributed to their deaths.

After the shooting, the agents' weapons were stolen. Peltier, along with others, was later arrested in connection with the shootings. Various evidence and eyewitness accounts connected Peltier to the events, and during this time, he provided several alibis regarding his activities. His trial revealed complex forensic evidence, and he was ultimately convicted of their murders in April 1977.

Peltier's conviction was followed by numerous appeals and claims of unfair trial practices, with some supporters raising doubts about his guilt. Despite this controversy, Peltier remained in prison, where his health declined over the years.

In a significant moment of his life, Peltier was sentenced to serve two consecutive life sentences. His appeals for clemency garnered support from various civil rights advocates and groups, but he faced continuous legal battles regarding his status as a political prisoner. Eventually, after decades of incarceration and advocacy, Peltier’s life sentence was commuted to home confinement in early 2025, marking a shift in his long legal ordeal.