Following Oklahoma's victory in the 2000 national championship, Roy Williams entered the 2002 NFL Draft as the leading figure among collegiate defensive backs. The consensus All-American achieved a clean sweep of the Bronko Nagurski Trophy and Jim Thorpe Award in 2001, establishing himself as the country's most formidable enforcer. The Dallas Cowboys, aiming to make him the cornerstone of their secondary, traded up to pick the hard-hitting safety eighth overall.

Paired alongside veteran Darren Woodson as a rookie free safety in 2002, Williams wasted no time asserting his presence, earning All-Rookie honors while tying a franchise record with two interception returns for touchdowns. His true breakout arrived in 2003, when he logged 75 tackles and two picks to earn First-Team All-Pro honors, anchoring Bill Parcells' top-ranked defense and kicking off an impressive run of five consecutive Pro Bowl selections through 2007.

Williams was a throwback defensive back whose sheer physicality fundamentally changed how opposing receivers approached the middle of the field. Known for imposing, bone-jarring hits, his horse-collar tackling technique proved so punishing to ball-carriers that the NFL officially outlawed the move after the 2004 season—a mandate forever etched in league vernacular as the "Roy Williams Rule”.

Shifting permanently to strong safety in 2005, Williams continued to serve as a primary physical enforcer for Big D, putting together back-to-back five-interception campaigns in 2002 and 2006. However, as the league increasingly shifted toward high-flying passing attacks, his physical, run-support style faced mounting coverage challenges against speedier tight ends and slot receivers.

A forearm fracture derailed his 2008 season after just three games, prompting his release from Dallas the following spring. Williams spent two final seasons with the Cincinnati Bengals before hanging up his cleats.

He compiled 19 interceptions, 510 combined tackles, 6.5 sacks, 56 pass deflections, and three defensive touchdowns for Dallas.