Callahan is identified as one of the Northwest Mystics – along with Guy Anderson, Morris Graves, and Mark Tobey, who shared a muted palette and strong interest in Asian aesthetics.

[3] At that time he was painting in a realist style inspired by Thomas Hart Benton and the artists of the Ashcan School, but he was deeply impressed by the originality of Klee, Kandinsky, and Feininger.

"[2] In 1926 Callahan had his first one-man show at San Francisco's Schwabacher-Frey Gallery; the following year he began his world travels as a ship's steward, winding up back in Seattle in 1930.

[4] The Callahans developed friendships with Dr. Richard Fuller (founder of the Seattle Art Museum), Mark Tobey, Morris Graves, and other progressive-minded artists.

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.

Later that same year Life magazine ran an article with large color photos extolling Callahan, Graves, Anderson, and Tobey as the "Mystic Painters of the Pacific Northwest".

In writings and interviews he explained that he wasn't interested in symbolism; rather, he saw his work as being firmly rooted in nature and art history - as it plainly was through the early part of his career.

"[7] Over time, figurative elements - men, horses, trees, insects - disappeared from his work, in favor of pure abstraction, but still, said Callahan, "It is nature, with its unlimited varied form, structure, and color that constitutes the vital living force from which art must basically stem.

[3] The Seventies saw two unusual commissioned works: In 1972 he designed costumes and sets for the Seattle Repertory Theatre's production of Macbeth, and in 1976, the owner of Longacres racetrack asked him to do a series of paintings of horses for an on-site restaurant.

In 1984 Callahan moved back to Seattle and turned his artistic attention to urban life, in contrast with the sea and light studies that dominated his work during two decades at Long Beach.