The three-story Richardsonian building with stone foundation and brick exterior walls is nearly square in plan. The interior features an impressive three-story interior light well. The staircase and balustrades are carved with stylized ornament in the Eastlake tradition.

Upper story fenestration in the street facades is a rhythmic arcade in which linteled openings are aligned under varied groupings of round-arched third-story transoms. Piers between bays rise as uninterrupted vertical bands behind which the spandrels are recessed. This fenestration, linked at the top by arches, and the round-arched portal in the principal facade emulate the Richardsonian Romanesque Style.

The design clearly is related to the multi-storied Richardsonian Romanesque office buildings that were being erected in Seattle after the Great Fire of 1889. However rudimentary a version it may be, the Mason Block too appears to owe inspiration - at least indirectly - to this much-copied Chicago prototype. Prime examples of the Richardsonian style are emphasized by the use of molded flat faced terra cotta whose dark red tones strikingly contrasts with the brick facing.

Originally, four ground story shops with conventional cast iron fronts were on Harris Avenue - two on either side of the central portal. On 12th Street, a bay window and two lunettes to light the front corner shop broke the ground story wall; and there was another business front at the south end.

Shop fronts were restored along original lines, and the interior spaces around the light well were converted to shop and restaurant use. Perhaps the most obtrusive alteration is the addition of a deep paneled frieze atop the ground story, which extends to grade level as a round-arched frame around the central portals of either facade