Manchester City loaned Karius to Mainz 05 in August 2011, where he played for Mainz 05's reserve team, Mainz 05 II, in the Regionalliga.
Manchester City meminjamkan Karius ke Mainz 05 pada Agustus 2011, di mana dia bermain untuk tim cadangan Mainz 05, Mainz 05 II, in the Regionalliga.