Selected with the number 8 overall draft pick in 2002 by the Dallas Cowboys out of the University of Oklahoma, Williams would establish himself immediately as a fierce defender for Dallas. In his rookie campaign as a Cowboy, Williams started all 16 games for Dallas at the safety position. He picked off five passes, returning them for 90 yards. Two of those interceptions were returned for touchdowns. Number 31 also had his highest solo tackle number for Dallas in his first season, recording 88.
Williams talent was recognized the following season as number 31 was named to the Pro Bowl as well as a first-team All-Pro. Prior to his third season with Dallas, Williams suffered a herniated disc in training camp. He was still able to get on the field for all 16 regular season games for Dallas. Williams recorded 94 total tackles and was once again named to the Pro Bowl.
For five consecutive seasons as a Cowboy, Williams was selected for the Pro Bowl. The only two seasons he didn't receive the honor was as a rookie and in 2008, his final year with Dallas. Over seven seasons with Dallas, number 31 put up the following numbers:
- 19 Interceptions
- 56 Passes-defended
- 506 Total tackles
- 5-time Pro-Bowl selection