Mandron, 25, was offered a deal to stay at the Gills, but has decided to explore pastures new ahead of the new season.
The big striker signed for Steve Evans' side last summer on a free transfer following his release from Colchester United.
After impressing during pre-season on trial, Mandron signed a permanent contact at the Kent club and went onto make 28 appearances for Evans’ men, scoring six goals.
After playing some youth football in France, Mandron finished his academy years at Sunderland before signing a professional deal with the club.
He had loan spells at Fleetwood Town, Shrewsbury Town and Hartlepool United before moving permanently to Eastleigh, then Wigan Athletic, then Colchester, and then the Gills.
In total, he has 185 professional appearances to his name, and 35 goals.
Mandron was offered a new deal by Gillingham but has chosen to move on after just one year in Medway.
He will perhaps be remembered for netting a last-minute equaliser in Gillingham’s final game before the lockdown away to Sunderland, and he also put in a memorable performance at home to Lincoln City earlier on in the season.
The length of the deal has not been disclosed.
Mandron was beaming after signing for his new club, telling the club’s Twitter account: “The key factor was the philosophy of the club. It is a club that is really focused on playing good football, playing out from the back, so that was really the main thing for me.
“When I met with the manager, he went into really good detail about how they do things here and how he sees me in the team. It was really appealing.
📽 | MIKA: "I know it's a Club that love to play good football and that was the main thing for me."— Crewe Alexandra (Stay Alert🤷🏻♂️) (@crewealexfc) August 1, 2020
Hear what our second summer signing had to say on why he decided to turn down other Clubs to join the Alex 👇#CreweAlex pic.twitter.com/J5Xm3S0xTf
“I cannot wait to get started. It’s been so long since I played and trained properly so I’m looking forward to it.
Gillingham’s official Twitter account posted: “We would like to thank Mika for his services, and we wish him well for the future.”
🗞 | Mikael Mandron has signed for @crewealexfc ahead of the 2020/21 campaign.— Gillingham FC (@TheGillsFC) August 1, 2020
The striker scored six goals in 28 appearances for #Gills last season.
We would like to thank Mika for his services and we wish him well for the future. 👍#Gills https://t.co/y5R1JnMEWJ