Reiss was born and raised in the Black Forest of Germany. His father, a landscape painter, gave him his first lessons. His more formal education was in various of the art schools of Munich. The novels of James Fenimore Cooper deeply impressed Reiss, so much so, that he emigrated to the U.S. in 1913 with the express intention of painting American Indians. It took him several years of living in New York to save enough money (as a portrait, landscape and mural painter, art teacher and interior designer) to pursue this goal, but in 1919 he finally went West. He painted 81 commissioned portraits for the Great Northern Railroad, which were exhibited nationally and in Europe. He continued painting his vibrant images of the Indians of the Northwest for several decades and became friends with many of his subjects, particularly the members of the Blackfeet tribe, who initiated him into their tribe as `Beaver Child'. He also painted landscapes and other subjects. His preferred media were pastel and tempera. After his death, members of the Blackfeet scattered his ashes at the foot of the Rockies, near Glacier National Park.