There was no fee for the Nigerian international because his contract was up this summer with the German club.
The six- foot, three-inch, 29-year-old can play centre-half and at full-back and will bolster Albion's defensive options after Uwe Hünemeier's left to rejoin his former club SC 07 Paderborn, as well as iam Rosenior not being offered a new deal and Connor Goldson attracting interest from Rangers and their new manager Steven Gerrard.
International and Bundesliga experience
Born to a Nigerian father and a German mother, Balogun qualifies to play for Nigeria and he has made 16 appearances for them since his first cap back in 2014.
Balogun joined Mainz in 2015 and has made 41 appearances and scored one goal for them. However, his end to this season has been restricted by a rib injury.
However, he has now recovered from this and is in the preliminary 30-man Nigeria squad for the World Cup next month in Russia.
Brighton have fended off competition for the Nigerian defender from fellow Premier League side, Huddersfield Town.
More defence depth for the Seagulls
With the deal being a free transfer it means that Brighton have not had to throw out a huge transfer fee and have instead got themselves an international player, who is versatile for nothing meaning he will be a very handy option to manager Chris Hughton.
Speaking to the club's website, Hughton had the following to say on Balogun's signing: “I am delighted to welcome Leon to the club. He will add more depth to our defensive options, and I am pleased to get our first piece of business done so early in the summer.
“He is in the peak years of his career and I am sure he will integrate well into the squad," added Hughton. “He also has valuable experience, having played in the Bundesliga, the Europa League and at international level.”
The announcement of his signing means Balogun has now managed to sort out his future before the World Cup, so he can fully focus on the task in hand with Nigeria, rather than where he will be playing next season.