Thomas played for the Oakland Raiders for the entire duration of his professional football career between 1972 and 1977. He had back-to-back six-interception seasons in 1974 and 1975. Although it is commonly believed that his fierce tackling got him the nickname "Doctor Death," the truth (per John Madden's book Hey, Wait a Minute (I Wrote A Book!)) is that Hall of Fame lineman Bob "Boomer" Brown, when he first met him, thought that he looked like the cartoon character "Dr. Death," and the moniker stuck. Thomas died of a heart attack on July 24, 2011. He was 61.