Millen was an All-American at Penn State in 1978, and was selected by the Oakland Raiders in the second round of the 1980 draft. He played in the NFL for 12 seasons, primarily for the Oakland/Los Angeles Raiders, before spending the final three years of his career with the San Francisco 49ers and Washington Redskins. He won at least one Super Bowl with each team -- XV and XVIII with the Raiders, XXIV with the 49ers and XXVI with the Redskins -- and was a two-time All-Pro selection.