The City Hall Station in Manhattan was the beginning of the first New York City Subway. Now closed to the public, the station is used by local trains turning around on the IRT Lexington Avenue (No. 6) Line.

The City's first subway opened back in 1870, a short underground tunnel under Broadway that stretched 312 feet (95 m) from Warren Street to Murray Street near City Hall. Constructed by inventor Alfred Ely Beach, the editor of Scientific American, the subway was driven by pneumatic power. An eight-foot (2.4 m) long car that could carry 18 passengers was blown through the tunnel by a 100 horsepower (74.5 kW) fan; the blower was reversed to create a partial vacuum and suck the car back through the tunnel. Although Beach received a charter to extend the line from the Battery to Columbus Circle, the Panic of 1873 and innovations in electric traction motors left the pneumatic subway as a short-lived public demonstration project. Nonetheless, Beach's pneumatic subway helped to demonstrate the practicality of constructing an underground railroad in Manhattan.

NYC's first subway line included a 2,174-foot (663 m) long viaduct across the Manhattan Valley in Harlem.

The Tremont Street Subway in Boston opened in 1897 and was the first subway in the United States (although it was a streetcar line and not a rapid transit system). New York lost no time in advancing a plan for the construction of its own underground rapid transit system, adopting the alignment of first subway route that same year. Starting at City Hall in Manhattan, the first subway line that opened on October 27, 1904 ran north along Centre Street, Elm Street (now Lafayette Street), Fourth Avenue, and Park Avenue to Grand Central Station (the route used today by the No. 6 train), west along 42nd Street to Times Square (the route used today by the Grand Central Shuttle), and north along Broadway to 145th Street (the route used today by the No. 1 train).

South of 96th Street, the original subway line had four tracks with express stations located approximately 1.5 miles (2.4 km) apart at Brooklyn Bridge, 14th Street, Grand Central, 72nd Street, and 96th Street, while local stations were located approximately a quarter of a mile (400 m) apart. As station platforms were extended to accommodate longer subway trains, the original local stations at City Hall, Worth Street, and 18th Street were subsequently closed due to their proximity to nearby express stations.

The original subway line was extended northward along Broadway to the Harlem Ship Canal in 1906, to 225th Street in 1907, and to 242nd Street in 1908. In November 1904, a branch splitting off the line at 96th Street running north along Lenox Avenue to 145th Street opened (the route used today by the No. 3 train), along with another branch splitting off the Lenox Avenue Line to West Farms in the Bronx (the route used today by the No. 2 train). The subway line was extended southward down Broadway to South Ferry in 1905 and to Atlantic Avenue in Brooklyn in 1908 (the route used today by the No. 4 train). Including the extensions into the Bronx and Brooklyn, New York City's first subway line totaled 23.5 miles (38 km).

Once a local station on the original subway line, Times Square has grown to become the busiest station in the New York City subway system.

The awkward alignment of the route across 42nd Street was chosen because property owners opposed the construction of a subway on lower Broadway. Through trains across 42nd Street were discontinued in 1918 with the opening of the Interborough Rapid Transit (IRT) Seventh Avenue Line from the Battery to Times Square and the IRT Lexington Avenue Line from Grand Central to the Bronx. The expanded IRT system, with trunk lines running along the east and west sides of Manhattan and the 42nd Street cross-town shuttle was referred to as the "H" system given its appearance on a map.

After overcoming numerous legal and financial obstacles, construction of the first New York City subway officially started with a groundbreaking ceremony with Mayor Van Wyck in front of City Hall on March 24, 1900. The first section of the subway from City Hall to 145th Street was completed on October 27, 1904 and service was inaugurated with official ceremonies. It was not long before this new transportation facility was overtaken by the City's growth and more subways had to be built. To this day, the City has an active continuing reconstruction and rehabilitation program for updating its extensive accumulation of subway installations.

The first subway was a trailblazer in construction techniques. Concrete jack arches between steel beams and columns for subway roof and sidewalls, extensive use of reinforced concrete, shield driven underwater tunnels lined with cast iron segments, concrete-lined tunnels mined through earth and rock, and elevated steel structures set the pattern for successful rail rapid transit construction for many years. Many tall buildings had already been erected in lower and midtown Manhattan and construction methods had to be adapted to protect these buildings that lined the subway route. Maintaining street traffic, which included electric surface cars, required elaborate and substantial decking over the subway excavation. Networks of subsurface infrastructure—sewers, water mains, gas piping, electric power lines, telephone and telegraph conduits—had to be maintained and re-routed to make room for construction of the first subway line.

The majority of New York's first subway line was built near the surface using cut-and-cover construction and includes a reinforced concrete roof supported by closely-spaced steel I-beam columns.

Great strides were also made in the field of electric power generation. A large powerhouse built next to the West Side Highway between 58th and 59th Streets supplied the electric power needed to run the subway trains. Covering an area of 3 acres (1.2 ha), the massive powerhouse was designed in the Beaux-Arts style by McKim, Mead & White and used a total of 72 boilers to generate the steam necessary to drive twelve 6,700 horsepower (5,000 kW) generators. Electric power generation had progressed considerably since the turn of the century but at that time the power plant and eight substations along the subway route represented bold advances.

On the recommendation of the History & Heritage Committee, the ASCE Met Section Board of Directors voted in 1976 to nominate New York City's First Subway as a National Historic Civil Engineering Landmark. The original subway opened in 1904 following four years of construction. The Society's Board of Direction approved the designation of the subway as a landmark at the Spring 1977 meeting and the American Society of Mechanical Engineers joined ASCE in the landmark designation, jointly designating the pioneering New York City subway line a National Historic Civil and Mechanical Engineering Landmark.

A bronze plaque denoting the Societies' landmark designation was presented in a ceremony on February 1, 1978 in the Brooklyn Bridge-City Hall station on the IRT Lexington Avenue Line. The ceremony was conducted by Leonard Braun, Vice Chairman of the Metropolitan Transportation Authority and featured addresses by William R. Gibbs, President of the ASCE, S. Peter Kezios, President of ASME, Steven K. Kauffman, Executive Officer of the NYC Transit Authority, and a representative of the Mayor of the City of New York. The ceremonies concluded with a trip in vintage Brooklyn-Manhattan Transit (BMT) Standard subway cars to the New York Transit Museum at the old Independent Subway System (IND) Court Street station in Brooklyn.

The plaque marking the designation of the First New York City Subway as a National Civil and Mechanical Engineering Landmark.

The first New York City subway would lead to many innovations: introduction of the first all-steel subway car, installation of an efficient interlocking block system with overlapping track circuits and automatic trippers, which keep trains a certain distance apart, and the devising of a button that automatically stops a train once the motorman's hand is removed from the control panel.

A challenging engineering feat requiring three types of construction, the underground rapid transit lines of the IRT were later extended to the Bronx, Brooklyn, and Queens. The BMT entered into the subway field in 1913 and eleven years later the Board of Transportation was created to construct and operate the City's IND system, which opened in 1932. New York City acquired the IRT and BMT in 1940 and began the process of unifying the three systems into the world's largest subway system.