He was born Michel Bouyer in 1837. His father, Jean-Baptiste Bouyer, was a French Canadian who was employed by the American Fur Company, trading with Sioux in the Wyoming area. Mitch's mother was a Santee Sioux. His father was killed by Indians while trapping, about 1863. Mitch's Indian name was Kar-pash. He had three full sisters: Marie, Anne, and Thérèse, who seem to have been triplets born in 1840. He also had at least two half-brothers: John Bouyer (c. 1845-1871), who was hanged at Fort Laramie for killing an Army scout in the first legal execution in Wyoming Territory, and Antoine Bouyer (born 1852?), whom Walter Mason Camp interviewed in 1912. John, in an interview just before he was hanged, stated that there had been other siblings who had already died.