The original portion of the Eastern Parkway Line station's interior is a New York City designated landmark and listed on the National Register of Historic Places.

[11] The Borough Hall station opened on January 9, 1908, as the terminal for the extension of the IRT Lexington Avenue Line from Lower Manhattan.

[18][19] A temporary switch was installed just west of the station, allowing trains to terminate on the southbound track until the line could be extended.

[14][23] To address overcrowding, in 1909, the New York Public Service Commission proposed lengthening the platforms at stations along the original IRT subway.

[24]: 168  As part of a modification to the IRT's construction contracts made on January 18, 1910, the company was to lengthen station platforms to accommodate ten-car express and six-car local trains.

The Public Service Commission ultimately rejected a proposal for a Brooklyn Heights station because it would have required the tunnels to be built at an extremely steep slope of five percent.

[40] The eastern end was to contain stairs and escalators leading directly to Court Street and to the IRT Broadway–Seventh Avenue Line station.

[65] In 1922, the New York State Transit Commission directed its engineers to prepare plans for lengthening the platforms at 23 stations on the BMT's lines to accommodate eight-car trains.

[69] The New York City Board of Estimate appropriated $362,841 for the lengthening of the platforms at Court Street and five other stations in January 1926[70][71] and awarded the contract to Charles Meads & Company early the next month.

[78][79] As part of a Works Progress Administration program, the IRT entrance at Cadman Plaza and Montague Street was integrated into a neighboring park in late 1940.

Although ten-car trains already operated on the line, the rear car could not open its doors at the station due to the short platforms.

[89] In the late 1960s, New York City Transit extended the Fourth Avenue Line platform about 85 feet (26 m) to the west, allowing it to accommodate ten 60-foot (18 m) cars.

[91] In 1976, with funding from the Exxon Corporation, the Broadway–Seventh Avenue Line station, as well as three others citywide, received new "artfully humorous graffiti" murals and artwork.

[95][96] MTA chairman Richard Ravitch announced in October 1981 that these platforms would be renovated with funding from the Subway Committee for the Brooklyn Downtown Commercial Crescent, a local civic group.

[95][98] In 1982, the UMTA gave a $66 million grant to the New York City Transit Authority, part of which was allocated for the renovation of several subway stations, including Borough Hall's IRT platforms.

[102][103] Other parts of the renovation had been conducted haphazardly, such as the uneven installation of gray wall tiles,[103][104] as well as a ceiling that had been repainted above the platforms but not the tracks.

The MTA report suggested that special care be taken in the inspection of the thirteen subway stations that have terracotta ceilings due to the different properties when compared to concrete or steel.

[126] In November 2022, the MTA announced that it would award a $106 million contract for the installation of additional elevators at the Borough Hall station complex.

The IRT Broadway–Seventh Avenue Line station has two tracks and two side platforms that are stacked above each other, and it runs roughly northwest to southeast under Cadman Plaza and Borough Hall.

[158] Accessibility for the southbound Eastern Parkway Line platform was proposed in February 2019 as part of the MTA's "Fast Forward" program.

[164] On the northbound side, the overpasses have full height turnstiles leading to two staircases, one each flanking Brooklyn Borough Hall's main entrance on the northeast corner of Court and Joralemon Streets.

[165] The stairs flanking Borough Hall retain cast-iron hoods atop granite bases, which are part of the original design.

[117]: 6  On the southbound side, the overpasses have small turnstile banks, leading to a token booth and two staircases, going up to the southeast corner of Court and Joralemon Streets.

The mezzanine has a large set of doors leading into the Municipal Building (this entrance was closed in February 1996 due to security concerns), and a now-defunct bank teller window.

[117]: 4 The secondary fare control area for the IRT Eastern Parkway Line is at the extreme west end and is unstaffed.

[39] The local tracks of the line were originally planned to travel north under what is now Cadman Plaza West to the Brooklyn Bridge.

[39][11][176] Both platforms have their original IRT trim line and name tablets reading "BOROUGH HALL" in a serif lettering style.

[178] Dark blue I-beam columns line both platforms at regular intervals with alternating ones having the standard black station name plate in white lettering.

White-on-green tile plaques with the words "Borough Hall", containing red, green, blue, buff, violet, and pink mosaic borders, are also placed on the walls.

[191] Yellow I-beam columns line both sides of the island platform at regular intervals with alternating ones having the standard black station name plate in white lettering.