Lossl, 27, makes the switch from the Breton minnow to the Bundesliga's surprise sixth-place side, and will be at the Coface Arena through 2019, the club announced Thursday.
After the highly regarded Karius joined Liverpool in May on a five-year term, Mainz's No. 1 priority ahead of next season's Europa League campaign was finding a starting 'keeper, and the Rhineland side has done exactly that.
"The coaching team, particularly goalkeeping coach Stephan Kuhnert, kept an eye on Jonas Lossl in the second half of last season and closely monitored him," Mainz manager Martin Schmidt said about the once-capped Dane.
"Looking at his performances, his presence and his personality, we’re very convinced that he’s got the whole package and we therefore focused our efforts on him early on. He is a seasoned 'keeper who we think is capable of making further development."
Lossly will assume first-choice 'keeper duties for Schmidt's lot after starting 37 of Guingamp's 38 Ligue 1 matches last season, where he finished joint-fifth in clean sheets with 13.
In what may very well become a trend in German football this year, Mainz announced the signing on Twitter with a photo poking fun at second-tier side St. Pauli. The Hamburg club yesterday finalised a move for Marvin Ducksch, and in the absence of manager Ewald Lienen, had a staffer wear a mask with the gaffer's face.
Apparently Lienen was at the Coface welcoming Mainz's newest member.