Inside the building, Charles Rosen painted a map of the Hudson Valley on the lobby walls, in addition to some murals depicting historic events in the area.

To the southeast another Register-listed property, the 1709 stone Madam Brett Homestead, sits in the middle of a large woodlot on a block east of Teller Avenue.

Along the northeast corner is a driveway to the parking lot in the rear, with a small guardrail of metal set in fieldstone separating it from Veterans Street.

[2] Three short steps lead up to the main entrance, with paneled wooden double doors flanked by decorative lanterns and topped with an 18-light transom set in a lintel with keystone.

Within is a cast pewter eagle; below the words "United States Post Office Beacon New York" are spelled out in affixed metal letters in the entablature.

The marble floor, white with grey veins, is complemented by a six-foot high (2 m) dado of the same material around the room in which post office boxes are set along the south wall next to a single large customer window.

[2] Above the dado the plaster walls feature a large mural depicting a map of the Hudson Valley, showing county boundaries and scenic highlights.

[2] The city of Beacon was incorporated in 1913 following the merger of two small villages near the confluence of the Hudson and Fishkill Creek: Matteawan, in the area where the post office is presently located, named for a textile factory that used the creek for power; and Fishkill Landing, the area around Lower Main Street and the train station, one and a half miles (2.4 km) to the west on the river.

It took its name from the mountain rising to its east, which had been the site of a bonfire pile meant to be lit as a beacon warning troops of the Continental Army in the valley if British ships were coming upriver in an attempt to retake it during the Revolutionary War.

The Beacon post office exemplifies many of the Colonial Revival's prominent features: large multipaned sash windows, symmetrical composition, gabled roofs with cupola, and pediments with fanlights.

Most of them are elsewhere in Dutchess County, further north up the east side of the Hudson River, in Wappingers Falls (now used as that community's village hall), Poughkeepsie, Hyde Park and Rhinebeck.

Beacon's successful use of fieldstone may have inspired President Franklin D. Roosevelt, a native of Dutchess County, to dictate its use in the other four post offices, as it had been the building material of choice for the early Dutch settlers of the region, including his ancestors.

He had been retained by the Treasury Department's Supervising Architect to help with the increased workload brought on by the expansion of public building projects during Roosevelt's New Deal.

[8] Inside, Charles Rosen, who had been awarded the commission by the Treasury's Relief Arts Project (TRAP), painted the mural, one of 15 public artworks that program, which unlike the department's larger Fine Arts Section did not rely on competitions to select artists, sponsored in post offices around New York.

In addition to the large map of the Hudson Valley, Rosen and his assistant Clarence Bolton painted small landscapes of the area with titles like View of Beacon from the River, The Old Power House and Waterfall, and The Old Swedish Church.

Three years later Rosen was awarded the commission to paint similar landscapes of the Poughkeepsie area for that city's post office.