Knowles was first elected to the Wisconsin State Senate in 1940, becoming Majority Leader after only two years in office in 1943, but during World War II he took a break to serve as a lieutenant in the U.S. Navy aboard the "USS Nevada." Following his military service, Knowles resumed serving in the Wisconsin State Senate and returned to the leadership position for 5 additional legislative terms until his election as Lieutenant Governor in 1954. Knowles resigned his Senate seat and was succeeded by his brother, Robert, who won an April 1955 special election to finish the unexpired senate term.