In 1898, the modern City of New York was formed with the consolidation of Brooklyn (until then a separate city), the County of New York (which then included parts of the Bronx), the County of Richmond, and the western portion of the County of Queens. The opening of the subway in 1904, first built as separate private systems, helped bind the new city together. Throughout the first half of the 20th century, the city became a world center for industry, commerce, and communication.

In 1904, the steamship General Slocum caught fire in the East River, killing 1,021 people on board. In 1911, the Triangle Shirtwaist Factory fire, the city's worst industrial disaster, took the lives of 146 garment workers and spurred the growth of the International Ladies' Garment Workers' Union and major improvements in factory safety standards.

A man stands in front of a modern curved roof building.
UN Secretary-General Dag Hammarskjöld in front of the United Nations Headquarters building, completed in 1952
New York's non-white population was 36,620 in 1890. New York City was a prime destination in the early twentieth century for African Americans during the Great Migration from the American South, and by 1916, New York City had become home to the largest urban African diaspora in North America. The Harlem Renaissance of literary and cultural life flourished during the era of Prohibition. The larger economic boom generated the construction of skyscrapers competing in height and creating an identifiable skyline.

New York became the most populous urbanized area in the world in the early 1920s, overtaking London. The metropolitan area surpassed the 10 million mark in the early 1930s, becoming the first megacity in human history. The difficult years of the Great Depression saw the election of reformer Fiorello La Guardia as mayor and the fall of Tammany Hall after eighty years of political dominance.

Returning World War II veterans created a post-war economic boom and the development of large housing tracts in eastern Queens and Nassau County as well as similar suburban areas in New Jersey. New York emerged from the war unscathed as the leading city of the world, with Wall Street leading America's place as the world's dominant economic power. The United Nations Headquarters was completed in 1952, solidifying New York's global geopolitical influence, and the rise of abstract expressionism in the city precipitated New York's displacement of Paris as the center of the art world.

The Stonewall riots were a series of spontaneous, violent demonstrations by members of the gay community against a police raid that took place in the early morning hours of June 28, 1969, at the Stonewall Inn in the Greenwich Village neighborhood of Lower Manhattan. They are widely considered to constitute the single most important event leading to the gay liberation movement Wayne R. Dynes, author of the Encyclopedia of Homosexuality, wrote that drag queen were the only "transgender folks around" during the June 1969 Stonewall riots. "None of them, in fact, made a major contribution to the movement." Others say the transgender community in New York City played a significant role in fighting for LGBT equality during the period of the Stonewall riots and thereafter.