Cox was originally drafted by the Denver Broncos in the fifth-round (137th overall) of the 2010 NFL Draft, where he started nine of 15 games for the Broncos, collecting 55 tackles (54 solo), one interception and 14 passes defensed. After spending the 2011 season out of football, he has spent the last two seasons with the San Francisco 49ers and played in 25 games in a reserve role and on special teams. He was released by the 49ers this month on November 12.

He has played in 40 career games with nine starts, posting 71 tackles (66 solo), one interception, 16 passes defensed and two forced fumbles.