Ken Duken is the third child of film and stage actress Christina Loeb, his father is a doctor. Duken never attended a drama school but took courses in drama, including under James Reynold. The actor initially played various theatre roles, including in such plays as Der Besuch der alten Dame (The visit of the old lady), Das Haus in Montevideo (The house in Montevideo) and in various Shakespeare plays. He debuted as a film actor in 1997, playing a small role in the television crime film Blutiger Ernst alongside Nadja Uhl and Daniel Brühl.
Duken played his first major role on a movie screen in 1999, alongside Franka Potente and Heiner Lauterbach in Friedemann Fromm's Schlaraffenland (Land of Milk and Honey). That same year he starred in Miguel Alexandre's drama Gran Paradiso, which was nominated for the German Film Award. For this film, in which he plays Mark, a young wheelchair user, he completed three months wheelchair-training. In 2003 he played a major role in Tödlicher Umweg (Deadly detour) by Curt Faudon. In 2005 his career continued in the cinema, with Buket Alakus's award-winning tragicomedy Eine andere Liga (Another league).