League spokesman Brian McCarthy wrote Friday in an email to NFL.com and NFL Network that the face mask Mathis has used throughout training camp "was considered a prohibited non-standard customized face mask."
"A player would need a medical reason to wear this type of face mask," McCarthy added about Mathis' gear, which covers up just about everything on his face except his eyes.
Several NFL players, including Darnell Dockett of the Arizona Cardinals and DeMarcus Ware of the Dallas Cowboys, have tried similar face masks, and those also will be disallowed. However, McCarthy said Baltimore Ravens defensive end Chris Canty and New York Giants defensive end Justin Tuck "both applied for medical clearance for their face masks and it was granted."
Mathis used his Twitter account to explain the NFL's decision to deny his face mask.
"its official.... the league says i cant wear my facemask. I will return to my run of the mill grill," he wrote.
Mathis wasn't surprised by the decision. On Thursday, he used Twitter to explain that he "may not" win the battle with the league office and asked fans: "anybody got pics/suggestions of a face mask within the horizontal4 and vertical3 guidelines?"
One fan even responded by urging Mathis to file an appeal.
Mathis hasn't said what face mask he will use, but presumably, it will fall within the bounds of league rules.
The Associated Press contributed to this report.