Tunnell first played for the University of Toledo where he suffered a broken neck injury that was so severe the US Army and US Navy both rejected his attempts to enlist during World War II. He eventually was accepted by the US Coast Guard and spent two years of service there before returning to play for the University of Iowa football team. He started as a quarterback, halfback and on defense during his two years as a Hawkeye. He led the team in passing in the 1946 season and receiving during the 1947 season. He quit the team before the 1948 season in order to join the New York Giants.
Tunnell was undrafted after college, and began his pro career by hitchhiking across the country from Iowa to New York City to meet Jack Mara, son of Giants founder Tim Mara, and ask to try out for the team. In his Hall of Fame induction speech, Tunnell thanked the West Indian banana-truck driver who dropped him off near this Polo Grounds "appointment".
Tunnell played 14 years in the National Football League. He played his first 11 years with the New York Giants and the last three years with the Green Bay Packers. Tunnell was a nine-time Pro Bowl selection. He moved from the Giants to the Packers when Giants defensive coordinatorVince Lombardi took over the head coaching duties at Green Bay. He led the NFL in punt return yards twice, in 1951 and 1952 and played a then NFL record of 143 consecutive games. He became a defensive leader with the Packers, helping out rookies and was valued for his defensive experience.
He ended his career with a record 79 interceptions (since surpassed by Paul Krause), which he returned for 1,282 yards and 4 touchdowns, and 16 fumble recoveries, along with another 3,506 return yards and 6 touchdowns on special teams. He was elected as the first African American in the Pro Football Hall of Fame in 1967. Tunnell became a scout and assistant coach with the Giants, where he died from a heart attack during a practice session in 1975.