By the year 1643, there were 13 indigenous tribes living on Long Island: Canarsie, Rockaway, Matinecock, Merrick, Massapequa, Nissequoge, Secatoag, Seatauket, Patchoag, Corchaug, Shinnecock, Manhasset and Montauk.

They used canoes as a source of transportation, and since they lived by the shores, they went fishing.^{[better source needed]} The fishermen used bows, arrows, and hooks to catch seafood such as crabs, scallops, and lobster.^{[better source needed]} The farmers used fish for fertilizer and planted vegetables such as corn, beans, and squash, which were popular among the indigenous people. They were exceptional farmers; they had a great understanding of how the weather and soil affected the crops.^{[better source needed]} Many of them hunted animals, such as deer, raccoon, and turkey in the forest.

The government that they set up was a participatory democracy and there was an alliance between the tribes. Each tribe had their own territory and chief that was respected by other tribes. Prior to European contact, the Lenape people (named the Delaware by Europeans) inhabited the western end of Long Island, and spoke the Munsee dialect of Lenape, one of the Algonquian language family. The Lenape (who were part of the Shinnecock Tribe) practiced record keeping and used wooden tablets, trees, and stones to keep record. They also used wampum belts to write down important messages. They also used their wampum to trade with the Europeans. The Lenape people, in specific, were seen as peacemakers by other indigenous tribes, although they would defend themselves if necessary. The Europeans admired their friendliness and their skills in mediation.

Giovanni da Verrazzano was the first European to record an encounter with the Lenapes, after entering what is now New York Bay in 1524. The island's eastern portion was inhabited by speakers of the Mohegan-Montauk-Narragansett language group of Algonquian languages; they were part of the Pequot and Narragansett peoples inhabiting the area that now includes Connecticut and Rhode Island.

In 1609, the English navigator Henry Hudson explored the harbor and purportedly landed at Coney Island. Dutch explorer Adriaen Block followed in 1615, and is credited as the first European to determine that both Manhattan and Long Island are islands.

In 1655, the European settlers purchased land from the Indigenous people. They split the land amongst themselves and continued to search the island for more land for settlement. Other parts of indigenous land were bought, areas that are now known as Brookhaven, Bellport, and South haven. These purchases occurred on June 10, 1664, the exchange was four coats and what is now $16.25. The indigenous people did not fully understand that the European claimed ownership of the land. They thought that the exchange was made for them to share the land.

The white settlers and the indigenous people lived amicably together for a while. However, some of the white settlers did not trust some of indigenous people. During King Philip's War in 1675, the English governor of New York ordered all canoes that were east of Hell Gate to be confiscated. This was done to prevent the Indigenous people from helping their allies in the mainland.

After the Dutch began to move into Manhattan many of the indigenous people moved to Pennsylvania and Delaware. They felt pressure to move as they did not fit in with the European culture or religion at the time. Many of them who stayed behind died from smallpox.

Native American land deeds recorded by the Dutch from 1636 state that the Indians referred to Long Island as Sewanhaka (Sewanhacky and Sewanhacking were other spellings in the transliteration of Lenape).Sewan was one of the terms for wampum (commemorative stringed shell beads, for a while also used as currency by colonists in trades with the Lenape), and is also translated as "loose" or "scattered", which may refer either to the wampum or to Long Island. The name "'t Lange Eylandt alias Matouwacs" appears in Dutch maps from the 1650s. Later, the English referred to the land as "Nassau Island", after the Dutch Prince William of Nassau, Prince of Orange (who later also ruled as King William III of England). It is unclear when the name "Nassau Island" was discontinued. Another indigenous name from colonial time, Paumanok, comes from the Native American name for Long Island and means "the island that pays tribute."

The very first European settlements on Long Island were by settlers from England and its colonies in present-day New England. Lion Gardiner settled nearby Gardiners Island. The first settlement on the geographic Long Island itself was on October 21, 1640, when Southold was established by the Rev. John Youngs and settlers from New Haven, Connecticut. Peter Hallock, one of the settlers, drew the long straw and was granted the honor to step ashore first. He is considered the first New World settler on Long Island. Southampton was settled in the same year. Hempstead followed in 1644, East Hampton in 1648, Huntington in 1653, Brookhaven in 1655, and Smithtown in 1665.

While the English eastern region of Long Island was first settled by the English, the western portion of Long Island was settled by the Dutch; until 1664, the jurisdiction of Long Island was split between the Dutch and English, roughly at the present border between Nassau County and Suffolk County. The Dutch founded six towns in present-day Brooklyn beginning in 1645. These included: Brooklyn, Gravesend, Flatlands, Flatbush, New Utrecht, and Bushwick. The Dutch had granted an English settlement in Hempstead, New York (now in Nassau County) in 1644, but after a boundary dispute they drove out English settlers from the Oyster Bay area. However, in 1664, the English returned to take over the Dutch colony of New Netherland, including Long Island.

The 1664 land patent granted to the Duke of York included all islands in Long Island Sound. The Duke of York held a grudge against Connecticut, as New Haven had hidden three of the judges (John Dixwell, Edward Whalley and William Goffe) who sentenced the Duke's father, King Charles I, to death in 1649. Settlers throughout Suffolk County pressed to stay part of Connecticut, but Governor Sir Edmund Andros threatened to eliminate the settlers' rights to land if they did not yield, which they did by 1676.

All of Long Island (as well as the islands between it and Connecticut) became part of the Province of New York within the Shire of York. Present-day Suffolk County was designated as the East Riding (of Yorkshire), present-day Brooklyn was part of the West Riding, and present-day Queens and Nassau were part of the larger North Riding. In 1683, Yorkshire was dissolved and the three original counties on Long Island were established: Kings, Queens, and Suffolk.

Early in the American Revolutionary War, the island was captured by the British from General George Washington in the Battle of Long Island, a decisive battle after which Washington narrowly evacuated his troops from Brooklyn Heights under a dense fog. After the British victory on Long Island, many Patriots fled, leaving mostly Loyalists behind. The island was a British stronghold until the end of the war in 1783.

General Washington based his espionage activities on Long Island, due to the western part of the island's proximity to the British military headquarters in New York City. The Culper Spy Ring included agents operating between Setauket and Manhattan. This ring alerted Washington to valuable British secrets, including the treason of Benedict Arnold and a plan to use counterfeiting to induce economic sabotage.

Long Island's colonists served both Loyalist and Patriot causes, with many prominent families divided among both sides. During the occupation British troops used a number of civilian structures for defense and demanded to be quartered in the homes of civilians. A number of structures from this era remain. Among these are Raynham Hall, the Oyster Bay home of patriot spy Robert Townsend, and the Caroline Church in Setauket, which contains bullet holes from a skirmish known as the Battle of Setauket. Also in existence is a reconstruction of Brooklyn's Old Stone House, on the site of the Maryland 400's celebrated last stand during the Battle of Long Island.

In the 19th century, Long Island was still mainly rural and devoted to agriculture. The predecessor to the Long Island Rail Road (LIRR) began service in 1836 from the South Ferry in Brooklyn, through the remainder of Brooklyn, to Jamaica in Queens. The line was completed to the east end of Long Island in 1844 (as part of a plan for transportation to Boston). Competing railroads (soon absorbed by the LIRR) were built along the south shore to accommodate travellers from those more populated areas. For the century from 1830 until 1930, total population roughly doubled every twenty years, with more dense development in areas near Manhattan. Several cities were incorporated, such as the 'City of Brooklyn' in Kings County, and Long Island City in Queens.

Until the 1883 completion of the Brooklyn Bridge, the only means of travel between Long Island and the rest of the United States was by boat or ship. As other bridges and tunnels were constructed, areas of the island began to be developed as residential suburbs, first around the railroads that offered commuting into the city. On January 1, 1898, Kings County and portions of Queens were consolidated into the 'City of Greater New York', abolishing all cities and towns within them. The easternmost 280 square miles (730 km²) of Queens County, which were not part of the consolidation plan, separated from Queens in 1899 to form Nassau County.

At the close of the 19th century, wealthy industrialists who made vast fortunes during the Gilded Age began to construct large "baronial" country estates in Nassau County communities along the North Shore of Long Island, favoring the many properties with water views. Proximity to Manhattan attracted such men as J. P. Morgan, William K. Vanderbilt, and Charles Pratt, whose estates led to this area being nicknamed the Gold Coast. This period and the area was immortalized in fiction, such as F. Scott Fitzgerald's The Great Gatsby, which has also been adapted in films.

Charles Lindbergh lifted off from Roosevelt Field with his Spirit of Saint Louis for his historic 1927 solo flight to Europe, one of the events that helped to establish Long Island as an early center of aviation during the 20th Century. Other famous aviators such as Wiley Post originated notable flights from Floyd Bennett Field in Brooklyn, which became the first major airport serving New York City before it was superseded by the opening of La Guardia Airport in 1939. Long Island was also the site of Mitchel Air Force Base and was a major center of military aircraft production by companies such as Grumman and Fairchild Aircraft during World War II and for some decades afterward. Aircraft production on Long Island extended all the way into the Space Age – Grumman was one of the major contractors that helped to build the early lunar flight and space shuttle vehicles. Although the aircraft companies eventually ended their Long Island operations and the early airports were all later closed – Roosevelt Field, for instance, became the site of a major shopping mall – the Cradle of Aviation Museum on the site of the former Mitchel Field documents the Island's key role in the history of aviation.

From the 1920s to the 1940s, Long Island began the transformation from backwoods and farms as developers created numerous suburbs. Numerous branches of the LIRR already enabled commuting from the suburbs to Manhattan. Robert Moses engineered various automobile parkway projects to span the island, and developed beaches and state parks for the enjoyment of residents and visitors from the city. Gradually, development also followed these parkways, with various communities springing up along the more traveled routes.

After World War II, suburban development increased with incentives under the G.I. Bill, and Long Island's population skyrocketed, mostly in Nassau County and western Suffolk County. Second and third-generation children of immigrants moved out to eastern Long Island to settle in new housing developments built during the post-war boom. Levittown became noted as a suburb, where housing construction was simplified to be produced on a large scale. These provided opportunities for white World War II military veterans returning home to buy houses and start a family. In his 1966 book, My Private America (Moja prywatna Ameryka), Kazimierz Wierzyński, a Polish poet who could not go back to Poland after World War Two, describes Polish farmers living there, as "walking novels".

By the start of the 21st century, a number of Long Island communities had converted their assets from industrial uses to post-industrial roles. Brooklyn reversed decades of population decline and factory closings to resurface as a globally renowned cultural and intellectual hotbed. Gentrification has affected much of Brooklyn and a portion of Queens, relocating a sizeable swath of New York City's population. On eastern Long Island, such villages as Port Jefferson, Patchogue, and Riverhead have been changed from inactive shipbuilding and mill towns into tourist-centric commercial centers with cultural attractions.

The descendants of late 19th and early 20th-century immigrants from southern and eastern Europe, and black migrants from the South, have been followed by more recent immigrants from Asia and Latin America. Long Island has many ethnic Irish, Jews, and Italians, as well as an increasing numbers of Asians and Hispanics, reflecting later migrations.

Long Island is one of the most densely populated regions in the United States. As of the United States 2010 Census, the total population of all four counties of Long Island was 7,568,304, which was 39% of the population of the State of New York. As of 2017, the proportion of New York City residents living on Long Island had risen to 58%, given the 5,007,353 residents living in Brooklyn or Queens. Furthermore, the proportion of New York State's population residing on Long Island has also been increasing, with Long Island's Census-estimated population increasing 4.0% since 2010, to 7,869,820 in 2017, representing 39.6% of New York State's Census-estimated 2017 population of 19,849,399 and with a population density of 5,617.3 inhabitants per square mile (2,168.9/km²) on Long Island. Long Island's population is greater than 37 of the 50 U.S. states.

As of the 2010 census, the combined population of Nassau and Suffolk Counties was 2,832,882 people; Suffolk County's share being 1,493,350 and Nassau County's 1,339,532. Nassau County had a larger population for decades, but Suffolk County surpassed it in the 1990 census as growth and development continued to spread eastward. As Suffolk County has more than three times the land area of Nassau County, the latter still has a much higher population density and is growing faster in the 21st century, given its proximity to New York City. According to the U.S. Census Bureau's 2008 American Community Survey, Nassau and Suffolk Counties had the 10th and 26th highest median household incomes in the nation, respectively.

Population figures from the U.S. Census Bureau Census 2010 show that whites are the largest racial group in all four counties, and are in the majority in Nassau and Suffolk Counties. In 2002, The New York Times cited a study by the non-profit group ERASE Racism, which determined that Nassau and Suffolk Counties constitute the most racially segregated suburbs in the United States.

In contrast, Queens is the most ethnically diverse county in the United States and the most diverse urban area in the world.

According to a 2000 report on religion, which asked congregations to respond, Catholics are the largest religious group on Long Island, with non-affiliated in second place. Catholics make up 52% of the population of Nassau and Suffolk, versus 22% for the country as a whole, with Jews at 16% and 7%, respectively, versus 1.7% nationwide. Only a small percentage of Protestants responded, 7% and 8% respectively, for Nassau and Suffolk Counties. This is in contrast with 23% for the entire country on the same survey, and 50% on self-identification surveys.

A growing population of nearly half a million Chinese Americans now live on Long Island. Rapidly expanding Chinatowns have developed in Brooklyn (布魯克林) and Queens, with Chinese immigrants also moving into Nassau County, as did earlier European immigrants, such as the Irish and Italians. The busy intersection of Main Street, Kissena Boulevard, and 41st Avenue defines the center of Downtown Flushing and the Flushing Chinatown, known as the "Chinese Times Square" or the "Chinese Manhattan". The segment of Main Street between Kissena Boulevard and Roosevelt Avenue, punctuated by the Long Island Rail Road trestle overpass, represents the cultural heart of the Flushing Chinatown. Housing over 30,000 individuals born in China alone, the largest by this metric outside Asia, Flushing has become home to the largest and one of the fastest-growing Chinatowns in the world as the heart of over 250,000 ethnic Chinese in Queens, representing the largest Chinese population of any U.S. municipality other than New York City in total. Conversely, the Flushing Chinatown has also become the epicenter of organized prostitution in the United States, importing women from China, Korea, Thailand, and Eastern Europe to sustain the underground North American sex trade.Flushing is undergoing rapid gentrification with investment by Chinese transnational entities,

More recently, a Little India community has emerged in Hicksville, Nassau County, spreading eastward from the more established Little India enclaves in Queens. Rapidly growing Chinatowns have developed in Brooklyn and Queens, as did earlier European immigrants, such as the Irish and Italians. As of 2019, the Asian population in Nassau County had grown by 39% since 2010 to an estimated 145,191 individuals, including approximately 50,000 Indian Americans and 40,000 Chinese Americans, as Nassau County has become the leading suburban destination in the U.S. for Chinese immigrants. Likewise, the Long Island Koreatown originated in Flushing, Queens, and is expanding eastward along Northern Boulevard and into Nassau County.

Long Island is home to two Native American reservations, Poospatuck Reservation, and Shinnecock Reservation, both in Suffolk County. Numerous island place names are Native American in origin.

A 2010 article in The New York Times stated that the expansion of the immigrant workforce on Long Island has not displaced any jobs from other Long Island residents. Half of the immigrants on Long Island hold white-collar positions.

The Counties of Nassau and Suffolk have been long renowned for their affluence. Long Island is home to some of the wealthiest communities in the United States, including The Hamptons, on the East End of the South Shore of Suffolk County; the Gold Coast, in the vicinity of the island's North Shore, along Long Island Sound; and increasingly, the western shoreline of Brooklyn, facing Manhattan. In 2016, according to Business Insider, the 11962 zip code encompassing Sagaponack, within Southampton, was listed as the most expensive in the U.S., with a median home sale price of $8.5 million.