The area now designated as Nassau County was originally the eastern 70% of Queens County, one of the original twelve counties formed in 1683, and was then contained within two towns: Hempstead and Oyster Bay. In 1784, the Town of North Hempstead, was formed through secession by the northern portions of the Town of Hempstead. Nassau County was formed in 1899 by the division of Queens County, after the western portion of Queens had become a borough of New York City in 1898, as the three easternmost towns seceded from the county.
When the first European settlers arrived, among the Native Americans to occupy the present area of Nassau County were the Marsapeque, Matinecoc, and Sacatogue. Dutch settlers in New Netherland predominated in the western portion of Long Island, while English settlers from Connecticut occupied the eastern portion. Until 1664, Long Island was split, roughly at the present border between Nassau and Suffolk counties, between the Dutch in the west and Connecticut claiming the east. The Dutch did grant an English settlement in Hempstead (now in western Nassau), but drove settlers from the present-day eastern Nassau hamlet of Oyster Bay as part of a boundary dispute. In 1664, all of Long Island became part of the English Province of New York within the Shire of York. Present-day Queens and Nassau were then just part of a larger North Riding. In 1683, the colonial territory of Yorkshire was dissolved, Suffolk County and Queens County were established, and the local seat of government was moved west from Hempstead to Jamaica (now in New York City).
By 1700, virtually none of Long Island's area remained unpurchased from the Native Americans by the English colonists, and townships controlled whatever land had not already been distributed. The courthouse in Jamaica was torn down by the British during the American Revolution to use the materials to build barracks.
In 1784, following the American Revolutionary War, the Town of Hempstead was split in two, when Patriots in the northern part formed the new Town of North Hempstead, leaving Loyalist majorities in the Town of Hempstead. About 1787, a new Queens County Courthouse was erected (and later completed) in the new Town of North Hempstead, near present-day Mineola (now in Nassau County), known then as Clowesville.
The Long Island Rail Road reached as far east as Hicksville in 1837, but did not proceed to Farmingdale until 1841 due to the Panic of 1837. The 1850 census was the first in which the population of the three western towns (Flushing, Jamaica, and Newtown) exceeded that of the three eastern towns that are now part of Nassau County. Concerns were raised about the condition of the old courthouse and the inconvenience of travel and accommodations, with the three eastern and three western towns divided on the location for the construction of a new one. Around 1874, the seat of county government was moved to Long Island City from Mineola. As early as 1875, representatives of the three eastern towns began advocating the separation of the three eastern towns from Queens, with some proposals also including the towns of Huntington and Babylon (in Suffolk County).
In 1898, the western portion of Queens County became a borough of the City of Greater New York, leaving the eastern portion a part of Queens County but not part of the Borough of Queens. As part of the city consolidation plan, all town, village, and city (other than NYC) governments within the borough were dissolved, as well as the county government with its seat in Jamaica. The areas excluded from the consolidation included all of the Town of North Hempstead, all of the Town of Oyster Bay, and most of the Town of Hempstead (excluding the Rockaway Peninsula, which was separated from the Town of Hempstead and became part of the city borough). In 1899, following approval from the New York State Legislature, the three towns were separated from Queens County, and the new county of Nassau was constituted.
In preparation for the new county, in November 1898, voters had selected Mineola to become the county seat for the new county (before Mineola incorporated as a village in 1906 and set its boundaries almost entirely within the Town of North Hempstead), winning out over Hicksville and Hempstead. The Garden City Company (founded in 1893 by the heirs of Alexander Turney Stewart) donated four acres of land for the county buildings in the Town of Hempstead, just south of the Mineola train station and the present day village of Mineola. The land and the buildings have a Mineola postal address, but are within the present day Village of Garden City, which did not incorporate, nor set its boundaries, until 1919.
In 1917, the hamlet of Glen Cove was granted a city charter, making it independent from the Town of Oyster Bay. In 1918, the village of Long Beach was incorporated in the Town of Hempstead. In 1922, it became a city, making it independent of the town. These are the only two administrative divisions in Nassau County identified as cities.
From the early 1900s until the Depression and the early 1930s, many hilly farmlands on the North Shore were transformed into luxurious country estates for wealthy New Yorkers, with the area receiving the "Gold Coast" moniker and becoming the setting of F. Scott Fitzgerald's 1925 novel The Great Gatsby. One summer resident of the Gold Coast was President Theodore Roosevelt, at Sagamore Hill. In 1908, William Kissam Vanderbilt constructed the Long Island Motor Parkway as a toll road through Nassau County. With overpasses and bridges to remove intersections, it was among the first limited access motor highways in the world, and was also used as a racecourse to test the capabilities of the fledgling automobile industry.
Nassau County, with its extensive flat land, was the site of many aviation firsts. Military aviators for both World Wars were trained on the Hempstead Plains at installations such as Mitchel Air Force Base, and a number of successful aircraft companies were established. Charles Lindberg took off for Paris from Roosevelt Field in 1927, completing the first non-stop trans-Atlantic flight from the United States. Grumman (which in 1986 employed 23,000 people on Long Island) built many planes for World War II, and later contributed the Apollo Lunar Module to the Space program.
The United Nations Security Council was temporarily located in Nassau County from 1946 to 1951. Council meetings were held at the Sperry Gyroscope headquarters in the village of Lake Success near the border with Queens County. It was here on June 27, 1950, that the Security Council voted to back U.S. President Harry S Truman and send a coalition of forces to the Korean Peninsula, leading to the Korean War.
Until World War II, most of Nassau County was still farmland, particularly in the eastern portion. Following the war, the county saw an influx of people from the five boroughs of New York City, especially from Brooklyn and Queens, who left their urban dwellings for a more suburban setting. This led to a massive population boom in the county. In 1947, William Levitt built his first planned community in Nassau County, in the Island Trees section (later renamed Levittown; this should not be confused with the county's first planned community, in general, which is Garden City). While in the 1930s, Robert Moses had engineered curving parkways and parks such as Jones Beach State Park and Bethpage State Park for the enjoyment of city-dwellers, in the 1950s and 1960s the focus turned to alleviating commuter traffic.
In 1994, Federal Judge Arthur Spatt declared the Nassau County Board of Supervisors unconstitutional and directed that a 19-member legislature be formed. Republicans won 13 seats in the election and chose Bruce Blakeman as the first Presiding Officer (Speaker).
According to a Forbes magazine 2012 survey, residents of Nassau County have the 12th highest median household annual income in the country and the highest in the state. In the 1990s, however, Nassau County experienced substantial budget problems, forcing the county to near bankruptcy. Thus, the county government increased taxes to prevent a takeover by the state of New York, leading to the county having high property taxes. Nevertheless, on January 27, 2011, a State of New York oversight board seized control of Nassau County's finances, saying the wealthy and heavily taxed county had failed to balance its $2.6 billion budgets.