- 1900: The Greater New York Charter is adopted, consolidating the five boroughs of Manhattan, Brooklyn, Queens, The Bronx, and Staten Island into a single city.
- 1901: The New York Public Library opens to the public. It is the largest public library system in the United States.
- 1904: The New York City Subway opens, making it the world's first public rapid transit system.
- 1910: The Triangle Shirtwaist Factory fire kills 146 garment workers, mostly women and young immigrants, leading to major changes in fire safety regulations.
- 1911: The Williamsburg Bridge opens, connecting Brooklyn and Manhattan.
- 1912: RMS Titanic sinks after hitting an iceberg in the North Atlantic, with over 1,500 passengers and crew members killed, including many prominent New Yorkers.
- 1914: World War I begins, and New York City becomes a major center for war production and shipping.
- 1918: The Spanish flu pandemic hits New York City, killing over 50,000 people.
- 1919: The Harlem Renaissance begins, a cultural movement that celebrates African-American art, music, and literature.
- 1920: The 19th Amendment to the US Constitution is ratified, granting women the right to vote, and the presidential election is held, with Republican Warren G. Harding defeating Democrat James M. Cox.