The German born defender helped SV Darmstadt 98 to gain promotion to the top flight but will playing against his former side with Mainz.
The club announced the deal on their official Twitter page.
“I look forward to the task ahead. Mainz 05 has established itself in recent years in the Bundesliga,” Balogun told Mainz official website.0
“That’s a very good opportunity for me. I am very pleased and proud to be part of the future of the club.”
Leon Balogun previously had stints with Hannover 96, Werder Bremen and Fortuna Dusseldorf.