A self-taught artist, Callahan stated that he derived much of his early art training from his extensive travels in Mexico and Europe. During the 1930s he executed murals as part of the Federal Arts Project for post offices in Centralia, and Anacortes, Washington; and Rugby, North Dakota. His work in murals continued into the 1970s, with a number of commissions for public and corporate buildings around Washington. His somber-toned abstracted figural compositions are widely represented in major American collections. His awards included a Guggenheim Memorial Fellowship in 1954 and a grant from the National Institute of Arts and Letters in 1968.
Callahan taught at Syracuse (New York) University--in the process of which he executed the mural The Cycle for the University--Boston University, and the Skowhegan School of Painting and Sculpture in Maine. As a critic, his articles appeared in the Seattle Times, Art News, and the American Magazine of Art, among other publications. He served on the Washington State Arts Commission, was a trustee of the Seattle (Washington) Museum, and an honorary trustee of the Washington State Museum.