Mertes was the 24th head football coach at Kansas State University in Manhattan, Kansas, and he held that position for five seasons, from 1955 until 1959. His record at Kansas State was 15–34–1.[1]
Drake
After leaving Kansas State following the 1959 season, Mertes became the 19th head football coach at Drake University in Des Moines, Iowa, serving for five seasons, from 1960 until 1964. His record at Drake was 27–19.[2]
Denver Broncos
After leaving Drake following the 1964 season, Mertes joined the Denver Broncos of the American Football League as an assistant coach. He coached with Denver for two seasons.