He played as an offensive tackle in the National Football League (NFL), primarily for the Philadelphia Eagles.

Thomas was selected by the Philadelphia Eagles in the first round with the 11th overall pick in the 1998 NFL draft.

Thomas was signed by the San Diego Chargers to a one-year contract on June 9, 2010.

Thomas rejoined the Philadelphia Eagles as a coaching intern in 2013, working primarily with the offensive line.

In April 2008, he decided to return to Tra Thomas after the name William failed to catch on.