Mathenia joined SV Darmstadt 98 in 2014 from 1. FSV Mainz 05 II on a free transfer. He made his 2. Bundesliga debut on 3 August 2014 against SV Sandhausen in a 1–0 home win. He went on to become first-choice keeper for the remaining season featuring in all of the 35 games in the league and cup keeping the most clean sheets in the process.
Mathenia joined Hamburger SV in 2016 from SV Darmstadt 98.