The dutchman signed a contract until the summer 2018 after his Galatasaray deal expired.
After signing the deal, De Jong claims he is a fan of the german clubs history.
“I like Mainz’s story, he said.
“The club has established itself in the Bundesliga off its own back.”
The Dutchman was strongly linked with a move back to Ajax, but Mainz acted efficiently as they looked to fill the void left by Fabian Frei’s departure.