Between the years 1909 and 1912, Wagner worked mostly as a furniture painter or 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. When the United States entered World War I, he joined the Army's first motion picture unit. He photographed several battles and was discharged in 1919. He continued working through the teens and 1920s as a gag writer. He also worked as an assistant director and second unit man for such directors as Allan Dwan and Lewis Milestone.
In the mid-1920s he wrote gags for Harry Langdon and Will Rogers. When talkies arrived, Wagner found himself making the difficult transition from silents. He never achieved the success he found in silents. Yet he found steady work as a gag man, adding bits of dialogue to comedy and dramatic films. In 1934, he helped script The Little Minister with Katherine Hepburn. He also helped write the short film La Cucaracha, which garnered RKO Pictures an Academy Award.
Even after this success, he struggled with putting pen to paper. When he came up with the story idea for A Medal for Benny he again was troubled with putting the story to paper. He had known John Steinbeck for many years. Steinbeck had considered Jack's mother, Edith Wagner, as his first writing coach while growing up in Salinas. Jack and his brothers -- Max, Blake and Bob -- had been steady friends and drinking buddies with Steinbeck since the 1920s. After much work Steinbeck and Jack wrote the script for Benny and it earned an Academy Award nomination for Best Original Story. It was the highlight of Jack's career. He went on to help Steinbeck with another script on The Pearl. He closed out his career as a producer of Mexican films featuring such actresses as Dolores Del Rio, among others. He died in Los Angeles.