Horning was a graduate of the School of Architecture of the University of California. (Most of the twenty art directors at MGM had architectural training.) Horning's career in Hollywood extended from 1935 till his death. He received two posthumous Academy Award nominations in a single year, for North by Northwest and Ben Hur. He won the Oscar for Ben Hur. He had previously been nominated for Gigi (1958).
For The Wizard of Oz, Horning and his staff had to design and build the sixty-five sets used in the film, along with the special properties, from moving talking trees to the Wicked Witch's giant hourglass.
According to assistant Jack Martin Smith, Horning was brilliant and supremely knowledgeable in technical matters. "It was the lunchtime game among the art directors to try to ask Horning a question he couldn't answer....You couldn't stump him."