Born in Portland, Oregon, Shannon Stirnweis settled in New York City, where he earned a reputation for his illustration work and fine art painting. He studied at the University of Oregon, the Art Center College of Design in Los Angeles, and with Pruett Carter.

He served in the U.S. Army as an illustrator and had special assignments in a variety of places that included the Berlin Wall, the Everglades, Arizona, and Alaska. He was also an illustrator for Sportsman Magazine, and Field and Stream as well as for books, posters, and ads. After serving two years as President of the Society of Illustrators in New York he entered the field of gallery painting. His skills as an illustrator served him well when he focused later on the history of the Old West.