Tammany Hall, also known as the Society of St. Tammany, the Sons of St. Tammany, or the Columbian Order, was an American political organization founded in 1786 and incorporated on May 12, 1789, as the Tammany Society. It became the main local political machine of the Democratic Party and played a major role in controlling New York City and New York state politics. Though initially an independent social organization, at its peak, Tammany Hall became synonymous with the New York County Democratic Party.

At its founding, the Society of St. Tammany was a social fraternal organization and one branch of a network of Tammany societies throughout the new United States of America. Politically, its members were Jeffersonian republicans opposed to the Manhattan aristocracy. As the immigrant population of New York grew, Tammany Hall became an important social and political organization, for Irish Catholic immigrants in particular. Following the 1854 mayoral election and the resulting mayoralty of Fernando Wood, Tammany Hall controlled Democratic Party nominations and political patronage in Manhattan for over a century through its organized network of loyal, well-rewarded, and largely Irish Catholic district and precinct leaders. It also gained support from the New York City business community for its efficient, if corrupt, solutions to problems.

At its peak, Tammany Hall also played a major role in state and national politics, particularly during the Gilded Age, when New York was sharply contested as a swing state, and it hosted the 1868 Democratic National Convention. Its positions typically represented the interests of its immigrant, ethnic, and Catholic voter base, in addition to the personal interests of its leadership. Prominent members or associates of Tammany included Aaron Burr, Martin Van Buren, Fernando Wood, Jimmy Walker, Robert F. Wagner, and Al Smith.

The Tammany Hall organization was also a frequent vehicle for political graft, most famously during the leadership of William M. Tweed, whose 1873 conviction for embezzlement gave the organization its national reputation for corruption. In the following decades, many reformist New York politicians developed national reputations opposing or criticizing Tammany influence, including Samuel J. Tilden, Grover Cleveland, Theodore Roosevelt, Franklin D. Roosevelt, Fiorello La Guardia, Robert Moses, Thomas E. Dewey, Jacob Javits, and Ed Koch.

Tammany Hall declined during the twentieth century, following the 1898 consolidation of greater New York City, which forced it to compete directly with other local organizations, and decades of sustained opposition from reform activists and changing demographics in Manhattan. It was dissolved in 1967.

Before 1898, New York City was coterminous with the island of Manhattan and New York County. As the population center of New York state, one of the largest states in the new United States of America, in addition to its status as a commercial center for the continent, struggles for control of city, county, state, and national offices were fierce.

The 1686 charter divided the city into six wards and established a Common Council consisting of an alderman and assistant alderman from each ward. Until 1938, wards were the smallest political units in New York City and the building block of its political organizations.

Under the 1686 charter, the mayor of New York City was appointed by the state government. In 1821, the Common Council was given authority to elect the mayor until, in 1834, the state constitution was amended to require the election of the mayor by direct popular vote.