Thomas attended Lanier High School in Macon. After playing at Florida State, Thomas was the Steelers' first-round selection (24th overall) in the 1973 NFL draft. That year, Thomas played in all 14 regular-season games and in the Steelers' only playoff game. He recorded one interception that season.
In 1974 and 1975, the Steelers won back-to-back Super Bowls. In 1974, Thomas picked up a career-best five interceptions and he scored a touchdown on a fumble recovery. Thomas was named to the Pro Bowl after a 1976 season in which he collected two interceptions and a fumble recovery. In 1978, he sat out the season due to a blood disorder known as Boeck's sarcoid.
During his time in Pittsburgh, Thomas played alongside cornerback Mel Blount. NFL.com named Thomas and Blount the sixth-best cornerback combination of all time.
During the 1982 preseason, Thomas was traded to the Denver Broncos for an initially undisclosed draft slot.