Cox attended University High School in Waco, Texas, where he was a standout on both sides of the football. As a senior, Cox recorded 27 tackles and led the team with nine interceptions. On offense, he had 47 catches for 670 yards and 12 touchdowns. He earned first-team Class 4A all-state honors, first-team all-Central Texas honors, and played in the 2006 U.S. Army All-American Bowl. Also a standout track & field athlete, Cox was one of the state's top performers in both sprints and jumps. As a sprinter, Cox had a personal-best time of 21.55 seconds in the 200-meter dash, and in jumps, he posted bests of 7.56 meters in the long jump and 14.60 meters in the triple jump. Considered a four-star recruit by Rivals.com, Cox was listed as the No. 8 cornerback prospect in the nation. He chose Oklahoma State over LSU, Nebraska, and Oklahoma.
College career
In his true freshman year at Oklahoma State, Cox became the first Cowboy since Barry Sanders to open a season with a kickoff return for a touchdown when he went 96 yards against Missouri State. Playing both on defense and special teams, Cox had 24 tackles on defense while breaking up five passes. His 595 kickoff return yards in 2006 represent the third best single-season total in school history. Cox became a starter during his sophomore year, as he started the last eight games of the regular season at cornerback. He tied for second on the team with two interceptions while breaking up eight passes, and was also credited with 42 tackles. Cox averaged 21.5 yards on 31 kickoff returns and his 631 kickoff return yards are second best for Oklahoma State in a single season. As a junior, Cox again returned the first kickoff of the season 90 yards for a touchdown against Washington State. He finished the season with 32 tackles, eight pass break-ups, and two interceptions. Cox entered his senior season just 110 kickoff return yards short of the Big 12 career record. In November 2009, he was named one of the twelve semifinalists for the 2009 Jim Thorpe Award.
Cox was considered one of the best cornerbacks available in the 2010 NFL Draft. His draft stock dropped after a slow 40-yard dash at the NFL Combine, but he was able to improve his workout numbers at OSU's Pro Day.
Denver Broncos
Cox was drafted by the Denver Broncos in the fifth round of the 2010 NFL Draft with the 137th overall pick. He played one year for the team and was released on September 3, 2011.
Cox signed with the San Francisco 49ers on a two-year deal on March 13, 2012. At the end of the 2012 season, Cox and the 49ers appeared in Super Bowl XLVII. In the game, he contributed on special teams, but the 49ers fell to the Baltimore Ravens by a score of 34–31. Cox was released by the 49ers on November 12, 2013 to make room on the active roster for returning linebacker Nick Moody.
Cox was signed by the Seattle Seahawks on November 26, 2013. He was released by the Seahawks on November 27, 2013. On December 11, 2013, Cox re-signed with the Seahawks, after a season-ending injury to safety Jeron Johnson. He was once again released by the Seahawks on December 27, 2013.
On March 14, 2015, Cox signed a three-year, $15 million contract with the Tennessee Titans. On November 28, 2016, Cox was released by the Titans.
Seattle Seahawks (second stint)
On January 18, 2017, Cox signed with the Seahawks after an injury suffered by DeShawn Shead following the 36–20 loss to the Atlanta Falcons in the Divisional Round of the 2016 playoffs. On May 9, 2017, he was released by the Seahawks.
Legal issues
In August 2009, Cox was arrested for driving with a suspended license. He was also suspended from the Cotton Bowl in his senior season by head coach Mike Gundy for violating the team curfew rule. Cox was arrested on December 9, 2010, for felonious sexual assault and taken to a Douglas County jail. He was released on December 10 on a $50,000 bond. His sexual assault charges were classified as class three and class four felonies which carry a sentence of two years to life in prison. He faced a possible sentence of ten years to life if convicted. However, on March 2, 2012, Cox was found not guilty of sexual assault charges in Douglas County, Colorado criminal court.
