He played college football for the NC State Wolfpack, and was selected first overall by the Houston Texans in the 2006 NFL draft.

He also played running back as a senior because of injuries at that position, rushing for 590 yards and three touchdowns on 58 carries.

He was just one of three North Carolina High School players to be named to FoxSN's 2002 Countdown to Signing Day All-South team.

[5][6] On October 1, 2006, Williams recorded his first sack and a half against Miami Dolphins' quarterback Daunte Culpepper in the Texans' first win of the season (score 17–15).

Three weeks later, Williams sacked Jacksonville Jaguars quarterback Byron Leftwich and recovered a fumble in the Texans' 27–7 upset.

Williams made five tackles, two sacks, and returned a fumble by Kansas City Chiefs fullback Kris Wilson for a touchdown.

The 2007 season was a year that Williams answered many of the critics that he gained because of his lofty top draft pick status.

This effort included three sacks and one forced fumble in the Texans' first ever Monday Night Football game on December 1, 2008.

On December 15, 2010, the Texans announced they were placing Williams on injured reserve for the remainder of the season with an inflamed hernia.

On October 12, the Texans placed Williams on injured reserve for the second consecutive year, ending his season.

[24] In 2012, Williams debuted a honey-toasted oats breakfast cereal called "MariO's", and the proceeds from the sales are intended to be donated to a Buffalo charity supporting children with cancer and their families.

[25] Williams resided in Orchard Park, New York during his time with the Bills, where he bought a 9,389-square-foot (872.3 m2) home for $2 million in 2012.

[30] This moniker is a hat tip to the popular Nintendo character Wario who is portrayed as having a similar bruising style and relentless demeanor as Williams.