The south wing has two elliptical domes designed by Alfred Mullett, as Supervising Architect of the Treasury, and they are over two of the eight granite monumental circular stairwells in the building. These domes are in the oldest wing of the EEOB, which was constructed between 1871 and 1875 to house the Department of State. Staff first moved into the wing on July 14, 1875.

Mullett laid out the building's floor plan with a dome-covered stairwell at each corner and a rotunda-covered double stairwell in the middle of the east and west sides. Mullett designed these corner domes to bring natural light deep into the building down each four-story high staircase beneath the domes.

An engineering and construction marvel, each solid granite step extends out from the wall over the staircase and is self-supporting. The stair railing is made of solid bronze balusters topped by a mahogany cap. There are 4,004 bronze balusters in the building, and these were an important responsibility of the cleaning crews to polish every night in the early years of the building.