It is located at the corner of Fourth Avenue and Union Street in Brooklyn, New York City, serving the communities of Park Slope, Gowanus and Carroll Gardens.

The station opened on June 22, 1915, as part of the initial portion of the BMT Fourth Avenue Line to 59th Street.

The Union Street station was constructed as part of the Fourth Avenue Line, the plan for which was initially adopted on June 1, 1905.

[19] In the 1960s, the NYCTA started a project to lengthen station platforms on its lines in Southern Brooklyn to 615 feet (187 m) to accommodate 10-car trains.

[6] On July 14, 1967, the NYCTA awarded a contract to conduct test borings at eleven stations on the Fourth Avenue Line, including Union Street, to the W. M. Walsh Corporation for $6,585 (equivalent to $60,172 in 2023) in preparation of the construction of platform extensions.

The NYCTA spokesman stated that the old tiles were in poor condition and that the change was made to improve the appearance of stations and provide uniformity.

As part of the modification, new graffiti resistant ceramic tile was installed, signage, advertising panels, benches and trash bins were removed and reinstalled, exhaust systems were added in the porter rooms at both platforms, and the wall-mounted conductors board was replaced by a ceiling-mounted one.

[31][32] In 2019, development firm Avery Hall Investments offered to pay $11 million for a new entrance to the southbound platform, including an elevator, as part of the development of a 17-story building on the northwestern corner of Fourth Avenue and Union Street (651 Union Street).

[34][35] In late March 2022, 204 4th Avenue LLC submitted an application for the new easement entrance to the New York City government.

[43][8]: 854  The walls were intended to improve ventilation, as passing trains would push air forward, rather than to the sides of the tunnel.

[46] Each platform has one same-level fare control area in the center and there are no crossovers or crossunders to allow free transfer between directions.

[47] The Manhattan-bound side has a fare control area, a turnstile bank, token booth, and two street stairs.

The Bay Ridge-bound side has a fare control area, a turnstile bank without a token booth, and two street stairs.