Born in Monterey, California, United States, Wagner was one of five children, all boys, of William Wallace Wagner, a railroad conductor, and Edith Wagner, a writer who provided dispatches for the Christian Science Monitor during the Mexican Revolution. He lived in Mexico from about 1895 to 1909 before moving to Los Angeles to work for D. W. Griffith on his early films. Between the years 1909 and 1912, Wagner worked mostly as a furniture painter, set designer and second unit cameraman. He then turned his attention to gag writing and found a job with Mack Sennett writing gags for Keystone Kops shorts. His specialty was comedy construction, especially the famed car chase scenes. He often alternated between writing comedy and working behind the camera as a principal photographer or assistant cameraman.
