His first dabbling in advertising was indicative of his skill for successful copywriting. P.F. Collier's published a series of books selected by the president of Harvard, Dr. Charles W. Eliot. The Harvard Classics series was selling poorly despite a series of ads running in Colliers Weekly. With a quarter page of free space in the magazine, Barton quickly put together an ad with the headline "This is Marie Antoinette riding to her death." The ad went on to tell of the practical value of being well read. Barton sold 400,000 sets of the Harvard Classics. The premise was simple an sincere, the style that made Bruce Barton famous.In 1915, Barton became editor of Every Week magazine. For three years, he became well known and liked by readers for his inspirational editorials. Again, his sincerity and ability to reach readers as if he were addressing them personally was his strength. By 1918, this popularity had become great enough that Redbook magazine hired him to write editorials exclusively for them.
During World War I, his talent for persuasive prose was put to use for campaigns to promote war movements. For the Salvation Army, he wrote the famous slogan, "A man may be down but he is never out." For the Victory Loans program, he created the ad "I am New York and this is my creed" with illustrator John Pennell. Among his most important engagements of the war was the 1918 planning of the United War Work Campaign.