According to Stephen Holder of the Indianapolis Star, Mathis made the decision more than a month ago:
Mathis shared his decision process with reporters, via WTHR's Bob Kravitz:
“I want to walk away, not limp away," Mathis said, per Kevin Bowen of Colts.com. "The rest of my body goes to my kids."
"You don't want to be pushed out," Mathis continued, per Kravitz. "Leave on your own merit."
Mathis posted a video on Instagram to thank Colts fans for their support over the years:
Colts wide receiver Donte Moncrief posted a farewell to Mathis on Twitter:
Mathis spent his entire career with the Colts, developing into the team's most feared pass-rusher with 122 career sacks, which ranks No. 1 in franchise history and No. 18 in NFL history.
The 35-year-old recorded 11 seasons in which he rang up seven or more sacks, and he led the league with 19.5 quarterback takedowns in 2013.
But he lost the following season to an Achilles injury, which looked as though it spelled the beginning of the end of Mathis' career.
He managed to come back strong in 2015 to put up seven sacks before mustering only four in 2016. He suffered a torn biceps in Week 12 that held him out for two games.
It wasn't the best output for an aging defensive end who was set to hit free agency, as his four-year, $36 million deal expires at the end of the 2016 season, per Spotrac.
Toward the end of the season, he wasn't sure of his plan, but he was prepared for whichever path he took, as he told Holder: "In a perfect world, I want to go out like Ray [Lewis], doggone it. But at the same time, it's been a great 14 years. A blessing. I can't complain about nothing. Nothing."
With that in mind, Mathis can ride off into the sunset with a Hall of Fame resume and the knowledge that not many other players will put on the Colts uniform and perform at his level.