After working at the Cologne Opera House under Eugen Szenkar, the Ulm City Theatre under Herbert von Karajan and Koblenz, he was engaged in 1937 by the Intendant and General Music Director Rudolf Krasselt at the Staatsoper Hannover as first lyrical baritone.
His talent and wit made him a paradigm actor of Rossini's The Barber of Seville, a role he has sung over 200 times and with which he has performed in Berlin, Düsseldorf, Hamburg and Cologne.
These two roles, for which his stature seemed less suitable, he often managed to escape with success by cunningly recommending his Berlin colleagues Karl Schmitt-Walter at the Intendance as guests for Don Giovanni or the Count.
The baritones Leonhard Delany, Tonio Larisch, Georg Schulz, Manfred Ball, Reinhard Braun and the tenors Horst Hoffmann as well as Joachim Siemann were also instructed in singing by him.
Several later university professors received their vocal training from Köhler: Manfred Ball, Günter Binge, Gerd Nienstedt, Friedrich-Wilhelm Tebbe and Michael Temme, and the concert and opera singers Christine Reil, Heike Henkel, Dieter Miserre and Ulf Kenklies.