Cornwell was an untiring worker who made many preliminary sketches before attempting a final painting, usually in oils. His work often appeared in most important magazines, and he also illustrated several books, including The City of the Great King, The Man of Galilee, Never the Twain Shall Meet, The Enchanted Hill and the Pride of Palomar. Cornwell was also President of the Society of Illustrators (1922-26). In the 1930s and 40s Cornwell concentrated on producing advertising posters.
Cornwell also painted several important murals. Notable among them were those for the Los Angeles Public Library, The Lincoln Memorial in Redlands, California, The Tennessee State Office Building, Eastern Airlines in Rockefeller Center, and the Raleigh Room at the Hotel Warwick in New York City.
Cornwell taught at the Arts Students League in New York. He was elected the Society of Illustrators Hall of Fame in 1959, one year before his death.