The stopper is the godson of Leicester legend Emile Heskey, and came through the club's academy.
He even captained the Under-23s at one stage, but never made an appearance for the first team.
After signing with Fleetwood in League One, Johnson said: "I feel very happy with the move - I'm delighted to get this opportunity and just want to come in now and hit the ground running.
"I spoke to the manager before joining about the history of the club, his targets for the year, what he wants to do. When he spoke to me about taking my career to the next level, I think Fleetwood is the perfect platform for me to push on with my career.
"I feel like last year I learned a lot from my experience in League One whilst at Wigan and Wimbledon. I learned what to do where and how to manage games, and how demanding Saturday-Tuesday games can be - I feel like all of that has helped me become a better player."
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