Westerman, who began his career as a country singer, appeared in over 50 films and televison productions, including Dances with Wolves, Hidalgo, The Doors, and Poltergeist, and Northern Exposure. He appeared in 12 episodes of the 1990s TV series, Walker, Texas Ranger, as Uncle Ray Firewalker.
As a young man, he was educated at the Wapheton and Flandreau Boarding Schools, where he became a close companion and life-long friend of Dennis Banks. He left his home on the Lake Traverse reservation in South Dakota, with a suitcase and an old guitar in hand. He rambled across the country playing country music and original tunes in bars and clubs, living for some time in Denver. In 1969, his first album Custer Died for Your Sins became the background theme of the emerging Red Power Movement.
As a member of American Indian Movement, and a spokesman for the International Indian Treaty Council, Westerman traveled the world extensively working for the betterment of native people. His vision of improved social conditions for indigenous people around the globe is reflected in the music of his second album, The Land is Your Mother, 1982. In 2006, he won a NAMMY Award for his third album, A Tribute to Johnny Cash. During his career, he played and collaborated with a number of notable musicians including Willie Nelson, Kris Kristopherson, Buffy St. Marie, Jackson Browne, Harry Belafonte, and Sting.