After studying law at the University of Vienna, Karl Ecker had theatre training at the k. k. Akademie für Musik und darstellende Kunst (Imperial Royal Academy of Music and Performing Arts) and private lessons with Burgtheater actors Ernst Arndt and Hans Siebert. He worked originally as a dramaturge and actor, from 1913 to 1917 also as director at the Deutsches Schauspielhaus in Hamburg under Max Grube, 1919/1920 at the Vereinigtes Deutsches Theater in Brünn / Brno under Rudolf Beer and from 1920 to 1922 at the Wanderbühne des Österreichischen Volksbildungsamtes (Travelling Theatre of the Office for Popular Education) under Friedrich Rosenthal, until he came in 1922 to the Urania in Vienna, where he was head of the administration and also deputized for the artistic director. As his influence was restricted because of financial difficulties, he left in 1934 and in 1936 became a research assistant in the National Library's Theatre Collection. He was mainly involved at the outset in exhibitions, such as the 1936 International Theatre Exhibition, and in the library, but during the Nazi era he was a member of the National Library branch of the SA dealing with the salvaging and storage of artworks and, from 1940, managing the library's administrative air raid protection agendas. It was in this connection that he was exempted from military service in 1942. Ecker's contract with the National Library ended in March 1950.