In 1917, Duer enlisted in the army and served in France with the fortieth engineers, working with camoflage. After the end of the war, he went on detached service in France and Germany, where he traveled in advance of occupying troops to copy German camoflage before it's destruction. Duer stayed in touch with several Pyle students during the war, especially Leslie Thrasher.
Duer returned to the United States in 1919, weakened by a severe illness. Yet, he resumed his art work and began the ten most productive years of his illustrating career. At the end of 1922 or the beginning of 1923, he moved to New York and set up a studio at 51 West Tenth Street. From 1919 to 1929 he illustrated for American Magazine, Country Gentleman, Everybody's, Pictorial Review, and many others. In 1928, Doer moved to Philadelphia. As well as working as an illustrator, he commuted weekly to Wilmington to teach at the Wilmington Academy of Art.With he Depression came a change in the market for illustration, Duer, like many other illustrators, was affected by the difficult times. He found work in public programs, specifically in the mural programs in Philadelphia and, later, the WPA. One of his morals was executed for the Philadelphia School Board at the J.S. Jenks School in Chestnut Hill, Philadelphia The artist also made dioramas at this time.