Johnson spent his entire career with the Panthers, who released him from his contract in February.
According to Max Henson of the Panthers' official website, Johnson said the back surgery he underwent last year set the wheels in motion for his eventual retirement: "Once you have back surgery, I don't care who you are, it's a beast to get back. ... I haven't worked out since this winter. I tried to get motivated to work out, and my body said, 'We're not working out right now.' It was about that time."
Johnson didn't record a sack last season in 12 games, marking the first time he failed to register at least one sack in a single campaign since he was a rookie in 2007.
The 32-year-old will finish his career with 67.5 sacks, which ranks second on the franchise's all-time list behind only Julius Peppers' 92.0 sacks.
From 2010-2014, Johnson racked up at least 8.5 sacks in each of those seasons, and he reached double digits on three occasions.
The 2007 third-round pick out of Georgia also recorded five sacks in seven career playoff games, and he was part of the Panthers team that lost 24-10 to the Denver Broncos in Super Bowl 50 at the conclusion of the 2015 season.