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 abook)") is that Hall of Fame lineman Bob Brown aka "The Boomer" who 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.