Throughout his artistic career, he participated in and helped define some important movements in American art, including regionalism, social realism, and caricature.

In Paris and Vienna he belonged to a group of expatriate intellectuals and artists, including Andrée Ruellan, Gertrude Stein, and ee cummings.

"[6] He supported himself through his time in Europe by providing light-hearted cartoons and scenic European landscapes to editors back in the U.S. A number of the caricatures he drew depicting the Roaring 20s, burlesque, opera houses, and the café scene appeared in such magazines as Vanity Fair.

[7] The fame that Dehn achieved during his time in Europe, his illustrations appearing in many leftist publications such as The Liberator, The New Masses, and The Dial, was noted by hometown paper The Minneapolis Journal in 1925.

[1] In New York, he began to focus his art on depicting scenes of Manhattan, showcasing the skyline and views of the city from the Staten Island Ferry.

[11] As the Great Depression had taken hold of the country, Dehn and his wife were desperately poor, and their financial difficulties contributed to their ultimate divorce.

During his period as a lithographer, his striking images of New York, including Central Park, captured the essence of the Roaring 20s and the 1930s Depression.

Beginning in 1930, Dehn made numerous trips back to his home in Minnesota, where he could live cheaply, and he executed a significant number of drawings and lithographs based on Midwest scenes.

He also summered on Martha's Vineyard from 1933 to 1936, often in the company of Thomas Hart Benton, Jackson Pollock, Georges Schreiber, and others in the vicinity of Gay Head and Menemsha, and joined by his girlfriend at the time, Eileen Lake.

In the early 40s, he worked as an instructor of etching and lithography at the Colorado Springs Fine Arts Center and received a citation from the U.S. treasury department for "Distinguished Service Rendered in Behalf of War Savings Program.

[13] He rose to the top tier of American watercolorists in short order, seen in a feature article on his landscape watercolors in Life magazine (August, 1941) and a traveling show organized by the Museum of Modern Art, "Four American Water Colorists" (1943–44) in which eleven Dehn watercolors were joined with the works of Winslow Homer, John Singer Sargent, and Charles Burchfield.

As well as visiting and painting Key West and the southwestern region of the United States, he went to Venezuela, Cuba, Haiti, Afghanistan and other areas of the world.