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.[2] 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.