Mills joined the Bills midway through the 2015 season from the Detroit Lions practice squad, going on to appear in 10 games while starting five. The 6-5, 316-pound Louisiana Tech product originally entered the NFL as a fifth-round pick (163rd overall) of the Chicago Bears in the 2013 NFL Draft. Mills spent both the 2013 and 2014 season with the Bears, starting all 29 games he appeared in.