Prieto was one of the most respected teaching potters of the craftsman years. He immigrated to the United States as a child in 1916. At age 14 he started working in a cigarette factory where he continued for seventeen years. He enrolled in ceramic classes in San Francisco before serving in the Army during WWII. When the war ended, Alfred University, New York (known for its ceramic program), accepted him without formal training in ceramics. In 1948, after two years at Alfred, Prieto accepted a teaching position at California College of Arts and Crafts (now California College of Arts), replacing Marguerite Wildenhain. In 1950, he succeeded Carlton Ball as head of the ceramics program at Mills College in Oakland where he taught until his death in 1967.
Prieto was keenly interested in the work of many artists and sought out the widest range of opportunity for interaction with personal travel and workshops at Mills College. In the early 1950s, he attracted graduate students such as Robert Arneson to Mills, and offered workshops around the country.