Lions president Tom Lewand said the team released Follett on Tuesday night.
The 24-year-old injured his neck in a helmet-to-helmet collision against the New York Giants in October and missed the remainder of the season. He did not participate in contact drills Tuesday, the first day the team was in full pads.
By Tuesday evening, Follett's Twitter and Web pages included tweets thanking coach Jim Schwartz and team executives before adding "Much as I fought to give it a go just wasnt in His plan. No longer a Lion."
Schwartz lauded Follett's efforts to return to football, but said he felt the Lions were deep enough at linebacker without him.
"To make the strides from being carted off the field and staying in New York in the hospital to getting back on the football field was a huge, huge step for him, and we've got a lot of respect for Zack," he said Wednesday. "But it's one of those situations with our numbers at linebacker that we felt this was the best for our football team."
Though he had just 31 tackles in two seasons, Follett endeared himself to fans off the field with a series of antics, including on his website, which featured video of him romping with a live lion and shopping for first-aid products for opposing ball carriers, all while wearing a helmet and street clothes.
Follett played in five of Detroit's first six games, starting two of them, before being injured last year. He played in a dozen games, mostly as a special teams player, in his rookie season.
Even after working to recover from the injury, Follett faced a daunting challenge just to make the Lions' linebacking corps. Detroit acquired free agents Stephen Tulloch and Justin Durant to complement DeAndre Levy. Free agent Bobby Carpenter, a seven-year veteran who played the final 10 games of last season with Detroit, signed after practice and will rejoin the team when the NFL year officially begins.
Cornerback Chris Houston signed a two-year contract and was on the field for the first time Wednesday. The five-year veteran started 15 games last season and is expected to do more of the same opposite free agent addition Eric Wright.
"It's where I'm comfortable," Houston said Wednesday. "It was really kind of a no-brainer. Detroit is an up-and-coming team.
"The defensive line helped sway the decision, too."
Like Carpenter and the rest of the team's free agents, Houston watched practice from the sideline Wednesday.
Lewand also said he expects restricted free agent defensive end Cliff Avril to sign a tender offer and be ready to join the team Thursday.
First-round draft pick Nick Fairley had surgery on his injured left foot Wednesday.
A source close to Fairley told ESPN.com senior NFL writer John Clayton that Fairley will miss three to four weeks.
The team says in a statement that he will miss a "significant portion" of training camp, but did not specify the type of surgery. The source also told Clayton it was not for a Lisfranc injury.
Information from The Associated Press contributed to this report.