By the time Cassidy was 20, he was an art director for a lithography firm in New York City. He already had a reputation as an expert draftsman and print maker. Cassidy’s journey out west started when he came down with a severe case of pneumonia that led to tuberculosis. He was given a six month life expectancy and traveled out west for the warm climate. During his stay in an Albuquerque sanatorium he did drawings of Indians for reproductions of postcards. Cassidy then moved to Denver where he found work as a successful commercial artist doing stage sets, entertainment posters, and magazine illustrations. Cassidy met his future wife Ina Sizer Davis in Denver. Through her encouragement, he made the leap to become a full time painter.
Cassidy’s art education started in his childhood town Cincinnati, Ohio. He studied at the Mechanic Institute in Cincinnati and with Frank Duvenck at the Cincinnati Art Academy.
Cassidy moved to New York and enrolled at the National Academy of Design and the Arts Students League. His respiratory illnesses brought an end to his education.
Cassidy met with Edgar L. Hewett the director of the School of American Archaeology.Hewett hired Cassidy to do a series of murals for the New Mexico Building at the 1915 Panama – California Exposition. Hewett wanted him to document the culture of Pueblo Indians. This commission gave Cassidy the motive to return to New Mexico with his new wife. They moved to Santa Fe in 1912 and Cassidy became a founding member of the Santa Fe Artists’ Colony. He was only the third Anglo artist to establish residency in Santa Fe.
Cassidy received recognition from his mural “Cliff Dwellers of the Southwest.” He was awarded the gold medal for his murals he did for the Panama- California Exposition. The Indians of the Southwest and their landscape became his touchstone for his work. He even went as far as to decorating his home with altar paintings from a ruined Nambe church.
In the 1920’s a benefactor sponsored a year of travel to Europe and North Africa. European museums bought two of his paintings. Picasso personally picked one of his paintings for the Luxembourg Palace in Paris.
Cassidy was hired by the WPA to paint murals in the Santa Fe Federal Courthouse. While working in close quarters, the paint fumes combined with turpentine and carbon monoxide led to Cassidy’s early death of lead poisoning.
His works are in major collections such as the Rockwell Museum of Western Art, NY, San Diego Museum, Ca, The Art Museum, Berlin, Germany and The Luxembourg in Paris.