The Callahans developed friendships with Dr. Richard Fuller (founder of the Seattle Art Museum), Mark Tobey, Morris Graves, and other progressive-minded artists. Their home became a meeting-place for Seattle's arts community, including prominent Japanese-American artists Kenjiro Nomura and Kamekichi Tokita, and many others.
In 1933 - at age 27 - he gained national recognition with the inclusion of some of his paintings in the First Biennial Exhibition of Contemporary Art at the Whitney Museum, in New York. The same year he began his twenty-year tenure with SAM, when it opened its new building in Seattle's Volunteer Park. Over the next two decades he curated exhibitions at SAM, wrote a weekly arts column for The Seattle Times, and took trips to Europe and Latin America; his main focus, however, remained his painting. He had numerous exhibitions, was commissioned to do several murals (including post office murals for the Federal Art Project in Anacortes and Centralia, Washington and Rugby, North Dakota), and helped form the Group of Twelve, an "independent salon" of Northwest artists. In the late 1930s he and his wife began spending much of their time in the Robe Valley area of the North Cascades mountains; during the Second World War he spent summers as a U.S. Forest Service fire lookout in the Cascades