Mewhort started all 45 games in which he played during his four NFL seasons, but knee injuries limited his playing time the past two seasons.
"I would like to thank the Irsay family and the entire Colts organization for giving me the opportunity to live a childhood dream," Mewhort said in a statement released by the team. "The fans and the city of Indianapolis have treated me like one of their own, and I am forever grateful. Wearing the Horseshoe was one of the biggest honors I have ever known, and I will always bleed blue. Go Colts!"
A second-round pick in the 2014 NFL draft by the Colts, Mewhort started 30 games in his first two NFL seasons.
But he played just 10 games in 2016, and played just five contests last season before being placed on season-ending injury reserve because of lingering problems with his knee.
"He fought through injuries and played through pain," general manager Chris Ballard said in a statement. "Jack is a selfless player who always carried a team-first mentality. He is revered among his coaches, teammates and staff and his leadership will be missed in the locker room. We're appreciative of Jack's dedication to the Colts and we wish him the best moving forward."
Mewhort signed a one-year, $1.5 million deal with the Colts in March, and in June, he said he expected to play at a high level this year.
There have been no recent reports regarding setbacks related to his knee, and it is unclear whether health was the primary reason for Mewhort's decision to retire.