Tammany Hall, also known as the Society of St. Tammany, the Sons of St. Tammany, or the Columbian Order, was a New York City 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 and helping immigrants, most notably the Irish, rise in American politics from the 1790s to the 1960s. It typically controlled Democratic Party nominations and political patronage in Manhattan after the mayoral victory of Fernando Wood in 1854, and used its patronage resources to build a loyal, well-rewarded core of district and precinct leaders; after 1850 the vast majority were Irish Catholics due to mass immigration from Ireland during and after the Irish Famine.

The Tammany Society emerged as the center for Democratic-Republican Party politics in the city in the early 19th century. After 1854, the Society expanded its political control even further by earning the loyalty of the city's rapidly expanding immigrant community, which functioned as its base of political capital. The business community appreciated its readiness, at moderate cost, to cut through regulatory and legislative mazes to facilitate rapid economic growth. The Tammany Hall ward boss or ward heeler, as wards were the city's smallest political units from 1786 to 1938, served as the local vote gatherer and provider of patronage. By 1872, Tammany had an Irish Catholic "boss", and in 1928 a Tammany hero, New York Governor Al Smith, won the Democratic presidential nomination. However, Tammany Hall also served as an engine for graft and political corruption, perhaps most infamously under William M. "Boss" Tweed in the mid-19th century. By the 1880s, Tammany was building local clubs that appealed to social activists from the ethnic middle class. In quiet times the machine had the advantage of a core of solid supporters and usually exercised control of politics and policymaking in Manhattan; it also played a major role in the state legislature in Albany.

Charles Murphy was the quiet but highly effective boss of Tammany from 1902 to 1924. "Big Tim" Sullivan was the Tammany leader in the Bowery, and the machine's spokesman in the state legislature. In the early twentieth century Murphy and Sullivan promoted Tammany as a reformed agency dedicated to the interests of the working class. The new image deflected attacks and built up a following among the emerging ethnic middle class. In the process Robert F. Wagner became a powerful United States Senator, and Al Smith served four terms as governor and was the Democratic presidential nominee in 1928.Tammany Hall's influence waned from 1930 to 1945 when it engaged in a losing battle with Franklin D. Roosevelt, the state's governor (1929–1932) and later U.S. President (1933–1945). In 1932, Mayor Jimmy Walker was forced from office when his bribery was exposed. Roosevelt stripped Tammany of federal patronage. Republican Fiorello La Guardia was elected mayor on a Fusion ticket and became the first anti-Tammany mayor to be re-elected. A brief resurgence in Tammany power in the 1950s under the leadership of Carmine DeSapio was met with Democratic Party opposition led by Eleanor Roosevelt, Herbert Lehman, and the New York Committee for Democratic Voters. By the mid-1960s Tammany Hall ceased to exist.