In May 1915, he was assigned to the Rome office of the Chicago Daily News, and there he interviewed Benito Mussolini, then a Socialist, who was urging Italy to enter the war on the side of the Allies.

In 1933, Mowrer won the Pulitzer Prize for Correspondence for his reporting on the rise of Adolf Hitler in Germany and was named president of the Berlin Foreign Press Association.

When the Chicago Daily News learned about the threats, Frank Knox, the owner of the newspaper, offered Mowrer a position in the paper's bureau in Tokyo.

After American diplomatic missions to Germany refused to guarantee his and his family's safety, and after a futile personal appeal to the newly appointed US ambassador to Germany William Dodd, Mowrer agreed to depart immediately,[2] in return for the release of Paul Goldmann, an elderly Jewish correspondent for the Austrian newspaper Neue Freie Presse, who was being held by the Gestapo for high treason.

Assigned in August 1940 to Washington, D.C. as a correspondent for the Chicago Daily News, Mowrer collaborated with William J. Donovan on a series of articles on fifth-column activities in Europe.

Several trips to the Far East in the next two years resulted in the book Global War: An Atlas of World Strategy (McClelland, 1942), which he wrote in cooperation with Marthe Rajchman.

Pursuing his ideas on international organization further in Challenge and Decision: A Program for the Times of Crisis Ahead (McGraw, 1950), Mowrer urged the United States to take the lead in forming a "peace coalition" and the ultimate federation of non-Communist countries, to weaken the "expansionist bloc."

He suggested that Soviet successes were compelling Western peoples to "pull themselves together in a real effort to survive as free men", and concluded that America's pioneer spirit was "still warm beneath the ashes of self-indulgence."