He shaped the artistic profile of the Musiktheater im Revier in Gelsenkirchen, nicknamed the Ruhr-Scala during his tenure, and the Hessisches Staatstheater Wiesbaden.

He was appointed Generalintendant (General Manager) of the Musiktheater im Revier in Gelsenkirchen on the recommendation of conductor Uwe Mund, and held the post until 1986.

[2] At the beginning of his tenure, he called the singer Carla Henius to form and direct the musik-theater-werkstatt (Music Theatre Workshop), which was devoted to the presentation of new operas.

Occasionally, notable singers were invited as guests to make performances more attractive, including Grace Bumbry, Helen Donath, Mirella Freni, Johanna Meier, Birgit Nilsson, Katia Ricciarelli, Leonie Rysanek, Theo Adam, Giacomo Aragall, Peter Dvorsky, Manfred Jung, Kurt Moll, Karl Ridderbusch, and Ingvar Wixell.

[2] Alluding to the noted Italian opera house La Scala, the theater was nicknamed the Ruhr-Scala during Leininger's tenure,[3] reflecting the upgrading of cultural offerings in what was essentially an industrial region along the Ruhr river.