His oil paintings were frequently featured in popular magazines and books as literary illustrations, advertisements, and posters promoting the war effort.

Brangwyn's style influenced Cornwell's artistic development in "rendering the human figure, bold outlines, flattened picture plane, and graphic approach to composition.

[6] Cornwell's paintings were in Cosmopolitan, Harper's Bazaar, Redbook, and Good Housekeeping magazines, illustrating the work of authors including Pearl S. Buck, Lloyd Douglas, Edna Ferber, Ernest Hemingway, W. Somerset Maugham, and Owen Wister.

Cornwell's February 1953 cover of a riverboat for True was later made into a U.S. Postage stamp as part of the USPS's 2001 American Illustrators series.

In 1927, the Los Angeles Public Library held a competition to paint the central rotunda of the building with murals depicting the history of California.

[3] By the time the U.S. entered WWII Cornwell was commissioned to create paintings of men in combat by the War and Navy departments, and by corporations like the Fisher Body company.

Cornwell also painted patriotic imagery for the Coca-Cola Company, the Pennsylvania Railroad, and did portraiture of wounded GIs as part of the U.S.O.

"[3] After World War II television became the more popular medium and people were less inclined to read short fiction from magazines, and heralded the end of the "celebrity illustrator.