The Cowboys wound up claiming Mills off of waivers from the Chicago Bears after he was among their final roster cuts.
Mills, 24, is in the third year of his four-year, $2,324,800 rookie contract that included a $164,800 signing bonus. He stands to make a base salary of $585,000 for the 2015 season.
The Bears cut Mills and S Sherrod Martin loose to make room for two players who were claimed off of waivers.
In 2014, Mills appeared in 13 games for the Bears and was rated by Pro Football Focus as the No. 66 offensive tackle out of 84 qualifying players.