Thomas' selection is the ninth in his nine NFL seasons and Mack will attend his third.
Players will be assigned to teams through the Pro Bowl Draft on January 27.
The 2016 Pro Bowl will be played on Sunday, January 31 in Honolulu, Hawaii.
Thomas ties the franchise record for Pro Bowl appearances, joining Browns legends Jim Brown (1957-65) and Lou Groza (1950-55, 1957-59).
Last season, Thomas became the first offensive lineman to make the Pro Bowl in each of his first eight seasons and joined a small group of NFL players at all positions who have accomplished the feat in the first nine.
"Jim Brown and Lou Groza are two of the greatest Browns of all time and the list of players who have been selected to the Pro Bowl their first nine seasons is a who's who of NFL history," Thomas said. "The neat thing about that list is that it spans many generations. To be included with names like those is pretty special and very humbling."
Mack, who scored a touchdown in the 2011 Pro Bowl, becomes the first center in Browns history to be selected to three Pro Bowls.
Since being chosen by Cleveland with the 21st overall pick of the 2009 draft, Mack has started 99 career games, the third-most by a Brown since the team's return in 1999.
Mack spent 11 weeks of the 2014 season on injured reserve. Since then, he has started and played every offensive snap in all 14 Browns games this season. Before fracturing his fibula in the second quarter of Cleveland's 31-10 victory over Pittsburgh in Week 6 last year, Mack started 85 games and played 5,279 consecutive offensive snaps to open his career.
"It's an honor to be voted to this year's Pro Bowl, especially coming off of a season where I had to watch from the sidelines for the first time," Mack said. "It was a lot of work during the offseason to come back from injury, but it was all worth it to be back on the field with my teammates. As a lineman, you depend on the man next to you, and I owe a lot of this honor to my teammates and coaches. Unfortunately, we haven't had the success that we wanted this year, but I look forward to representing Cleveland and the Browns in Hawaii."
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