NFL Network Insider Ian Rapoport, citing team sources, reported that Mack was not in the building to kick off the conditioning program.
While the workouts are completely voluntary, Mack's absence may serve as a means to deliver a message ahead of the final season of his rookie contract. The three-time Pro Bowl selection is set to make $13.846 million on the fifth-year option.
Mack, who is the 2016 NFL Defensive Player of the Year, likely is seeking a deal that makes him the highest-paid defensive end in the NFL.
The Las Vegas Review-Journal reported that the Raiders and Mack have been negotiating on a new contract.
Mack recorded 78 tackles and 10.5 sacks in 16 games last season. He has 303 tackles and 40.5 sacks in 64 career contests since being selected by the Raiders with the fifth overall pick of the 2014 NFL Draft.