Mitchell, 26, made the announcement at the University of Georgia's TEDxUGA event. The former University of Georgia star was a fourth-round pick of the Patriots in the 2016 draft.
In January, he told ESPN's Mike Reiss that he wasn't giving up hope of an NFL return despite a lingering knee injury and having to undergo 10 surgeries.
Mitchell hasn't played since his rookie season in 2016 when he played an instrumental role in the Patriots' comeback from a 28-3 deficit in Super Bowl LI, with five catches for 63 yards in the fourth quarter of that game.
He played in 16 games as a rookie (including playoffs) and totaled 39 receptions for 476 yards and four touchdowns. Because of the ongoing knee issues, he spent the entire 2017 season on injured reserve and was released last August.
A native of Valdosta, Georgia, who is an author of children's books, he has continued to be an advocate for the importance of reading.