The first post office in the Monona County seat of Onawa was established in 1855 and consisted of a few boxes in the corner of the Fairchild General Store. After locations in a number of local storefronts, bids were opened for a standalone post office building in May 1936. The contract was awarded to the H.W. Underhill Construction Company of Wichita, Kansas, on August 20, 1936. Work was completed at a cost of $49,720 in April 1937.

The WPA-era building’s lobby features an oil-on-canvas mural named Soil Erosion and Control. The mural, painted by Lee Allen, was carried out under the program of the “Section of Fine Arts, Federal Works Agency, Public Building Administrations,” which was established to decorate Federal buildings with murals and sculpture. Approximately 1% of the building cost was dedicated for this decoration.