Dorothy Straight and her husband, Willard Straight, were both deeply influenced by The Promise of American Life, a book written by the journalist, Herbert Croly. In 1914 Croly was invited to meet Dorothy and Willard at their Long Island home. While there, Croly commented that Norman Hapgood, the recently appointed editor of Harper's Weekly, had failed to turn it into the liberal journal that America needed. Dorothy suggested that the three of them should start their own journal.
The first edition of the New Republic appeared on 7th November, 1914. Willard Straight supplied the money and Herbert Croly became its first editor. The magazine was run by a small editorial board that included Croly's friend, Walter Lippmann. All outside contributions were submitted to the editorial board and had to be accepted by all members before it could appear in the magazine. Early contributors included Walter Weyl, Randolph Bourne, Charles Beard, Amy Lowell, Henry Brailsford and H. G. Wells.
When it was first published, the New Republic had 32 pages, including self-cover, and contained no illustrations. Its first edition sold 875 copies but after a year the circulation reached 15,000. The New Republic became a strong supporter of Theodore Roosevelt and the Progressive movement.
On the outbreak of the First World War, Herbert Croly argued for American neutrality. The New Republic published articles by British critics of the war such as Norman Angell and Harold Laski. However, after the sinking of the Lusitania, Croly urged American entry into war. After Congress declared war on Germany, the New Republic gave Woodrow Wilson its full support. This upset those that still believed in neutrality and Max Eastman, editor of The Masses, complained that the New Republic had become a mouthpiece of President Wilson.
After the war Herbert Croly became much more critical of Woodrow Wilson and described the Versailles Treaty as "a peace of annihilation". He also disliked the League of Nations, an organisation that "would perpetuate rather than correct the evils of the treaty." Sales of the New Republic reached 43,000 during the First World War but declined during the 1920s.
Willard Straight died during the influenza epidemic in 1918 but Dorothy Straight continued to fund what had now become a loss-making venture. Herbert Croly continued to persuade some of the most prominent literary figures in the United States and Britain to write for the journal. This included Edmund Wilson, Waldo Frank, Jane Addams, Bertrand Russell, H. G. Wells, Virginia Woolf and John Maynard Keynes.
Bruce Bliven replaced Herbert Croly as editor of the New Republic in 1930. Bliven continued the tradition of the New Republic to argue for left of centre solutions to America's problems and in 1932 supported the socialist candidate, Norman Thomas, for president. Four years later, Bliven switched to Franklin D. Roosevelt.
Writers who wrote for the New Republic between the wars included H. L. Mencken, John Dos Passos, Willa Cather and Michael Gold. In 1946 Henry A. Wallace became editor and under his leadership circulation reached a all-time high of nearly 100,000. Wallace resigned in December, 1947, when he decided to run for the presidency. He was replaced by Michael Whitney Straight, the son of the magazine's founders. Circulation of the New Republic fell to 30,000 in the 1950s and one commentator described it as "that faint voice of the left".