Williamson played one year for the Steelers in the National Football League in 1960. Next, he moved to the new American Football League. Williamson played four seasons for the AFL's Oakland Raiders, making the AFL All-Star team in 1961, 1962, and 1963. He also played three seasons for the AFL's Kansas City Chiefs. During his period of playing for the Chiefs, Williamson became one of football's first self-promoters, nurturing the nickname "The Hammer" because he used his forearm to deliver karate-style blows to the heads of opposing players, especially wide receivers. Before Super Bowl I, Williamson garnered national headlines by boasting that he would knock the Green Bay Packers starting receivers, Carroll Dale and Boyd Dowler, out of the game. He stated "Two hammers to (Boyd) Dowler, one to (Carroll) Dale should be enough".