Demtrøder has been active in German television, notably as a regular on Der Landarzt (The Country Doctor) as Wanja Teschner from 1992 until 2013, and a similarly long stint on the police series Großstadtrevier (1992-2010) as plainclothesman Henning Schulze. He was a regular on the soap opera Verbotene Liebe (2011-2014) and appeared in episodes of Leonie Löwenherz and Tatort.
Occasionally active in Hamburg dubbing, he was the German voice of Callan Mulvey on the Australian soap Heartbreak High and Nicky Katt in Way of the Gun. He narrates audio books and has been heard in the long-running Three Investigators series. For the 2002 video game Battlefield: 1942, he voiced the German soldier.