This station was constructed as part of the Dual Contracts by the Interborough Rapid Transit Company and opened in 1918.
Following the completion of the original subway, there were plans to construct a line along Manhattan's east side north of 42nd Street.
In July 1911, the IRT had withdrawn from the talks, and the Brooklyn Rapid Transit Company (BRT) was to operate on Lexington Avenue.
[9][10] It was predicted that the subway extension would lead to the growth of the Upper East Side and the Bronx.
Initially, service was provided only as a shuttle on the local tracks of the Lexington Avenue Line starting at Grand Central, continuing past this station and under the Harlem River to 167th Street on the IRT Jerome Avenue Line.
[18][19] In 1952 or 1953, a public address system was installed at this station, providing information to passengers and train crews.
In July 2023, as part of a pilot program to deter assaults on New York City Transit staff, the MTA painted "no standing zones" on the 125th Street station's platforms near the centers of each train.
It would be built underneath 125th Street, below and perpendicular to the existing Lexington Avenue Line station.
[34] When opened, it will initially be served by the Q train, with the T providing service when phase 3 of the line is built.
[47] On October 18, 2016, the de Blasio administration announced a rezoning plan for East Harlem.
In their request for funding, they cited that they wanted to avoid an uncertain response from the first administration of Donald Trump and start construction on Phase 2 as soon as possible.
[52] Under the approved plan, the MTA would complete an environmental reevaluation by 2018, receive funding by 2020, and open Phase 2 between 2027 and 2029.
[53] In January 2017, it was announced that Phases 2 and 3, which are expected to cost up to a combined $14.2 billion, were on the Trump administration's priority list of 50 most important transportation projects nationwide.
[54][55] In July 2018, the MTA released a supplemental environmental assessment for Phase 2 of the Second Avenue Subway.
[59] The July 2018 plans call for two tracks and one island platform, with switches both to the west and the east.
[60] Extra transfer capacity to the existing Lexington Avenue Line station would be provided as part of the construction of the Harlem–125th Street terminal.
In the 2018 report, the MTA stated that it also wanted to include a property on the intersection's southeast corner within the construction site.
[62] The station is mentioned by Lou Reed in his 1967 song "I'm Waiting for the Man", performed with The Velvet Underground, in which he describes traveling to Harlem to purchase heroin: "Up to Lexington / One-two-five / Feeling sick and dirty / More dead than alive.