The 18th Street station was constructed for the Interborough Rapid Transit Company (IRT) as part of the city's first subway line, which was approved in 1900.

Construction of the line segment that includes the 18th Street station started on September 12 of the same year.

[4]: 21  However, development of what would become the city's first subway line did not start until 1894, when the New York State Legislature passed the Rapid Transit Act.

It called for a subway line from New York City Hall in lower Manhattan to the Upper West Side, where two branches would lead north into the Bronx.

[4]: 161  The Rapid Transit Construction Company, organized by John B. McDonald and funded by August Belmont Jr., signed the initial Contract 1 with the Rapid Transit Commission in February 1900,[6] in which it would construct the subway and maintain a 50-year operating lease from the opening of the line.

[5]: 4  Belmont incorporated the Interborough Rapid Transit Company (IRT) in April 1902 to operate the subway.

East Side local trains ran from City Hall to Lenox Avenue (145th Street).

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

[10]: 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.

[13][14] The commission postponed the platform-lengthening project in September 1923, at which point the cost had risen to $5.6 million.

Additional columns between the tracks, spaced every 5 feet (1.5 m), support the jack-arched concrete station roofs.