The post office is a one-story, five-bay brick building with stone water table. Its double-doored main entrance is at the top of a set of granite steps with iron railings. The door is surrounded by fluted Doric pilasters and an entablature; above it is a fanlight with cast aluminum eagle. The words "UNITED STATES POST OFFICE" are spelled out in bronze letters across the frieze, with "DELMAR 12054 NEW YORK" above the door.[1]
On either side of the door are two original lamps and two windows with limestone sills and lintels. A wooden dentiled cornice runs around the roofline. Above it is a slate gabled roof.[1]
The side elevations have a pair of windows each. The gable ends are sided in clapboard with a semicircular opening with radiating mullions. At the rear are two wings: one three-bay workroom wing where the cornice is replaced with a parapet. A four-bay wing, added later, duplicates the roof detailing on the main building but is not considered contributing.[1]
On the inside, the entry leads to a wooden vestibule with paneled pilasters. The L-shaped lobby has a red-and-black terrazzo floor, white marble dado and dark marble baseboards. They give way to plaster, which forms a simple cornice at the junction of wall and ceiling. A 1940 mural, "The Indian Ladder", by Works Progress Administration artist Sol Wilson fills the wall above the postmaster's office. Many of the original furnishings remain.[1]
Delmar had had a post office since 1840, in space leased in a building on Elsemere Avenue. A single-family house, demolished after the new building was authorized in the mid-1930s, stood on the present site. General contractors Loucks and Clarke of Wallingford, Connecticut won the bid and broke ground in 1939, opening the new building the following year.[1]
The post office has seen only one significant change since then. The rear wing was built in 1959 to handle increased volume.[1]
Louis Simon, Supervising Architect of the Treasury Department, under whose jurisdiction the Post Office was at the time, designed 13 post offices in New York in a variation of the same basic model. Of these, Delmar's is the only one to not include a cupola in its original design (Attica's had one when built, but it has since been removed).[1]