Right after finishing secondary school in his hometown Munich, he attended a directing course at New York Film Academy which mainly raised his interest in acting.
He also participated in several films and series as a protagonist or guest actor, in numerous national projects and also in international co-productions, such as Minona, between Germany, France, and Russia, which tells the personal life of Ludwig van Beethoven.
In 1998, the actor financed his university studies by being part of the main cast in the prime time series Klinikum Berlin Mitte – Leben in Bereitschaft,[3] where he played Dr. Daniel Thies for three consecutive seasons.
He played Johann Hohenfels, a fictional character who flees the chaos of WWII and as a peace-loving pacifist travels the backlands of Brazil selling Aspirins to the poor by showing them commercial films through his movie projector, completely new for them at the time.
[12] In 2008 Ketnath worked again with Joseph Vilsmaier, participating in the main cast of the film The Legend of Brandner Kaspar [de],[13] based on a book by Wolfgang Franz von Kobell.
[14] The year 2009 saw the beginning of his work in the criminal series Stuttgart Homicide,[15] playing the lead male detective who sticks to his personal freedom and his own sense of justice.
In 2018, he came back to the stage and participated in Wolfgang Amadeus Mozart's opera Die Entführung aus dem Serail in the role of Bassa Selim, the emperor who finally frees his slave Konstanze.
He plays the antagonist Thomas Zumckle, a general of Hitler's elite troops who falls sickly in love with Aracy de Carvalho, a German-Brazilian secretary who saved numerous Jews from the Holocaust.