Duncan played with the American Football League San Diego Chargers from 1964 through 1969, and the NFL's Chargers in 1970 and Washington Redskins from 1971 through 1974. He was an AFL All-Star in 1965, 1966, and 1967 and an AFC-NFC Pro Bowler in 1971. Duncan now resides in Stockton, California. He previously worked at Webster Middle School, and is currently employed with Harrison Elementary School and other local elementary schools.