The transition tag for centers in 2014 is $10.039 million, about $1 million less than Mack would have made if he had been designated as Cleveland's franchise player.
By using the transition tag, however, Cleveland will not receive any draft-pick compensation if Mack signs elsewhere.
The decision also means Pro Bowl safety T.J. Ward is now a free agent.
Mack was the first draft pick in the Eric Mangini era, and he anchored the center of the line the past five seasons. His departure from the Browns would create a tough hole to fill given his consistency and dependability.
Mack has started in every game since he was drafted in 2009, rarely missing a snap. He was named to the Pro Bowl in 2013 and was voted the second-best center in the league on the AP All-Pro team (in a season that Maurkice Pouncey missed to injury).
Browns left tackle Joe Thomas raved about Mack late in the season, saying he is the best in the league at recognizing defensive fronts and making line calls.
"[Mack is] probably the best that I've seen and has to be one of the best in the NFL at understanding the mental side of the game and getting everybody on the same page blocking wise," Thomas said.
ESPN NFL Insider Adam Schefter, ESPN.com Browns reporter Pat McManamon and The Associated Press contributed to this report.