After graduating, Temerario began a coaching career, first at East Liverpool High School in Ohio and then as an assistant at Denison University and Indiana University in the 1930s and 1940s. His career was interrupted by service in the U.S. Navy during World War II. He returned to Indiana in 1945 as an assistant to Bo McMillin as the team won the Big Ten Conference championship. When McMillin became head coach of the Lions in 1948, Temerario moved with him. He worked there for two years before being hired as an assistant for the Browns in 1950. The Browns won the NFL championship that year.