When the first European settlers arrived, among the Native Americans to occupy the present area of Nassau County were the Marsapeque, Matinecoc, and Sacatogue.

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.

[17] 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.

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.

[31] The land and the buildings have a Mineola postal address, but are within the present day Village of Garden City,[32] which did not incorporate, nor set its boundaries, until 1919.

In 1917,[33] the hamlet of Glen Cove was granted a city charter, making it independent from the Town of Oyster Bay.

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.

[34] 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[35]) built many planes for World War II, and later contributed the Apollo Lunar Module to the Space program.

Council meetings were held at the Sperry Gyroscope headquarters in the village of Lake Success, near the border with Queens County.

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.

In 1994, Federal Judge Arthur Spatt declared the Nassau County Board of Supervisors unconstitutional and directed that a 19-member legislature be formed.

[39] Nassau County borders the Long Island Sound on the north and the open Atlantic Ocean on the south.

The county occupies a portion of Long Island immediately east of the New York City borough of Queens.

[40] Nassau County has a climate similar to other coastal areas of the Northeastern United States; it has warm, humid summers and cool, wet winters.

The Atlantic Ocean helps bring afternoon sea breezes that temper the heat in the warmer months and limit the frequency and severity of thunderstorms.

The nearly US$2 billion transportation infrastructure enhancement project was expected to accommodate anticipated growth in rail ridership and facilitate commutes between New York City and Nassau and Suffolk counties on Long Island.

[67] Likewise, the Long Island Koreatown originated in Flushing, Queens, and is expanding eastward along Northern Boulevard[68][69][70][71][72] and into Nassau County.

[66][69][70] The New York Times cited a 2002 study by the non-profit group ERASE Racism, which determined that Nassau, and its neighboring county, Suffolk, as the most de facto racially segregated suburbs in the United States.

The Nassau County Police Department posts the mug shots of DWI offenders as press releases on their website.

Nassau County is currently protected and served by 71 independent volunteer or combination paid/volunteer fire departments, organized into 9 battalions.

Smith succeeded Madeline Singas after she was nominated and confirmed as an associate judge on the New York Court of Appeals in June 2021.

The Chief Deputy is responsible for managing the activities of all departments of the Nassau County government, which provides services to its 1.36 million residents.

Liberal voters dominate many of the wealthy communities of the North Shore, particularly in the Town of North Hempstead where affluent villages such as Sands Point, Old Westbury, Roslyn, Kensington, Thomaston, Great Neck Plaza, and Great Neck Estates as well as the neighboring City of Glen Cove vote consistently Democratic.

In the western portion of the county, wealthy Garden City is solidly Republican, as is the middle-class community of Floral Park.

The Brooklyn Nets of the National Basketball Association, then known as the New York Nets, formerly played their home games in Nassau County at the now-demolished Island Garden arena in West Hempstead from 1969 to 1972 and then at the Coliseum from 1972 to 1977, before the franchise moved to New Jersey—its original home for several years before coming to Long Island in the late 1960s – and eventually, to Brooklyn.

Nassau County is also the home of the New York Lizards of Major League Lacrosse, who play at Shuart Stadium.

[91] In August 2024, Nassau County passed into law a ban on wearing face masks in public, making it a misdemeanor subject to a $1,000 fine and up to one year in prison to wear a facial covering in public, a move that was criticized by the New York Civil Liberties Union as a "dangerous misuse of the law to score political points.