In a statement posted to Johnson's Twitter account, he thanked his family and friends for their support and Mississippi State's players, trainers and fans, whom he credited for helping him build character and skills.
"With that being said, since I am not very tough, and Leach is glad I am leaving, I will be entering my name into the transfer portal with the hopes of finding a more fit playing environment for me," Johnson wrote.
Johnson's name hadn't been entered into the transfer portal as of Wednesday night, sources told ESPN.
Johnson, a junior from Greenville, Mississippi, didn't respond to a request for comment from ESPN. It seems he won't play against Illinois in the ReliaQuest Bowl in Tampa, Florida, on Jan. 2.
In a text message to ESPN on Wednesday, Leach wrote: "We only comment on players currently with our program. In any case, we wish him the best with his future."
Johnson ran 89 times for 488 yards with three touchdowns this season, while catching 48 passes for 285 yards. He had one of his best games of the season in the No. 22 Bulldogs' 24-22 victory at rival Ole Miss on Nov. 24, rushing for 73 yards on 12 carries and catching eight passes for 46 yards.
After that game, Johnson told reporters that the Bulldogs needed to be more balanced on offense. Mississippi State ranked next-to-last in the FBS in rushing with 78.8 yards per game. Only Boston College (63.3) was worse.
"I feel like we just need to run the ball," Johnson said. "That's just what it comes down to."
Johnson was the No. 41 athlete and No. 13 prospect from Mississippi in the 2020 ESPN 300.