The 35-year-old, who has also played for Stoke City, Burnley and Southampton, is to become an academy coach with the Major League Soccer Eastern Conference club.
City boss Carl Heeley told BBC Sport: "[Thomas] asked me if we would release him from his contract and obviously I'm going to do that. It's fully deserved. It's a wonderful opportunity that he couldn't turn down."
Thomas played his last game for the Faithful in the 1-1 draw with Guiseley at Aggborough earlier this month.