European colonization of the Bronx began in 1639. The Bronx was originally part of Westchester County, but it was ceded to New York County in two major parts (West Bronx, 1874 and East Bronx, 1895) before it became Bronx County. Originally, the area was part of the Lenape's Lenapehoking territory inhabited by Siwanoy of the Wappinger Confederacy. Over time, European colonists converted the borough into farmlands.

The development of the Bronx is directly connected to its strategic location between New England and New York (Manhattan). Control over the bridges across the Harlem River plagued the period of British colonial rule. The King's Bridge, built in 1693 where Broadway reached the Spuyten Duyvil Creek, was a possession of Frederick Philipse, lord of Philipse Manor. The tolls were resented by local farmers on both sides of the creek, and in 1759, Jacobus Dyckman and Benjamin Palmer led them in building a free bridge across the Harlem River. After the American Revolutionary War, the King's Bridge toll was abolished.

The territory now contained within Bronx County was originally part of Westchester County, one of the 12 original counties of the English Province of New York. The present Bronx County was contained in the town of Westchester and parts of the towns in Yonkers, Eastchester, and Pelham. In 1846, a new town was created by division of Westchester, called West Farms. The town of Morrisania was created, in turn, from West Farms in 1855. In 1873, the town of Kingsbridge was established within the former borders of Yonkers, roughly corresponding to the modern Bronx neighborhoods of Kingsbridge, Riverdale, and Woodlawn Heights.

Among famous settlers in the Bronx during the 19th and early 20th centuries were author Willa Cather, tobacco merchant Pierre Lorillard, and inventor Jordan L. Mott, who established Mott Haven to house the workers at his iron works.

The consolidation of the Bronx into New York City proceeded in two stages. In 1873, the state legislature annexed Kingsbridge, West Farms, and Morrisania to New York, effective in 1874; the three towns were soon abolished in the process.

The whole territory east of the Bronx River was annexed to the city in 1895, three years before New York's consolidation with Brooklyn, Queens, and Staten Island. This included the Town of Westchester (which had voted against consolidation in 1894) and portions of Eastchester and Pelham. The nautical community of City Island voted to join the city in 1896.

On January 1, 1898, the consolidated City of New York was born, including the Bronx as one of the five distinct boroughs (at the same time, the Bronx's territory moved from Westchester County into New York County, which already contained Manhattan and the rest of pre-1874 New York City).

On April 19, 1912, those parts of New York County which had been annexed from Westchester County in the past decades were newly constituted as Bronx County, the 62nd and last county to be created by the state, effective in 1914. Bronx County's courts opened for business on January 2, 1914 (the same day that John P. Mitchel started work as Mayor of New York City).Marble Hill, Manhattan was now connected to the Bronx, but it did not become part of that county by a historical accident due to changes in waterways.

The history of the Bronx during the 20th century may be divided into four periods: a boom period during 1900–29, with a population growth by a factor of six from 200,000 in 1900 to 1.3 million in 1930. The Great Depression and post World War II years saw a slowing of growth leading into an eventual decline. The mid to late century were hard times, as the Bronx changed during 1950–85 from a predominantly moderate-income to a predominantly lower-income area with high rates of violent crime and poverty in some areas. The Bronx has experienced an economic and developmental resurgence starting in the late 1980s that continues into today.

The Bronx was a mostly rural area for many generations, with small farms supplying the city markets. In the late 19th century, however, it grew into a railroad suburb. Faster transportation enabled rapid population growth in the late 19th century, involving the move from horse-drawn street cars to elevated railways and the subway system, which linked to Manhattan in 1904.

The South Bronx was a manufacturing center for many years and was noted as a center of piano manufacturing in the early part of the 20th century. In 1919, the Bronx was the site of 63 piano factories employing more than 5,000 workers.

At the end of World War I, the Bronx hosted the rather small 1918 World's Fair at 177th Street and DeVoe Avenue.

The Bronx underwent rapid urban growth after World War I. Extensions of the New York City Subway contributed to the increase in population as thousands of immigrants came to the Bronx, resulting in a major boom in residential construction. Among these groups, many Irish Americans, Italian Americans, and especially Jewish Americans settled here. In addition, French, German, Polish, and other immigrants moved into the borough. As evidence of the change in population, by 1937, 592,185 Jews lived in the Bronx (43.9% of the borough's population), while only 54,000 Jews lived in the borough in 2011. Many synagogues still stand in the Bronx, but most have been converted to other uses.

Bootleggers and gangs were active in the Bronx during Prohibition (1920–33). Irish, Italian, Jewish, and Polish gangs smuggled in most of the illegal whiskey, and the oldest sections of the borough became poverty-stricken.

Between 1930 and 1960, moderate and upper income Bronxites (predominantly non-Hispanic Whites) began to relocate from the southwestern neighborhoods of the borough. This migration has left a mostly poor African American and Hispanic (largely Puerto Rican) population in the West Bronx. One significant factor that shifted the racial and economic demographics was the construction of Co-op City, built with the intent of housing middle-class residents in family-sized apartments. The high-rise complex played a significant role in draining middle-class residents from older tenement buildings in the borough's southern and western fringes. Most predominantly non-Hispanic White communities today are located in the eastern and northwestern sections of the borough.

From the early 1960s to the early 1980s, the quality of life changed for some Bronx residents. Historians and social scientists have suggested many factors, including the theory that Robert Moses' Cross Bronx Expressway destroyed existing residential neighborhoods and created instant slums, as put forward in Robert Caro's biography The Power Broker. Another factor in the Bronx's decline may have been the development of high-rise housing projects, particularly in the South Bronx. Yet another factor may have been a reduction in the real estate listings and property-related financial services offered in some areas of the Bronx, such as mortgage loans or insurance policies—a process known as redlining. Others have suggested a "planned shrinkage" of municipal services, such as fire-fighting. There was also much debate as to whether rent control laws had made it less profitable (or more costly) for landlords to maintain existing buildings with their existing tenants than to abandon or destroy those buildings.

In the 1970s, parts of the Bronx were plagued by a wave of arson. The burning of buildings was predominantly in the poorest communities, such as the South Bronx. One explanation of what occurred was that landlords decided to burn their low property-value buildings and take the insurance money, as it was more lucrative to get insurance money than to refurbish or sell a building in a severely distressed area. The Bronx became identified with a high rate of poverty and unemployment, which was mainly a persistent problem in the South Bronx. There were also cases where tenants burnt themselves out of their apts/buildings to qualify themselves for emergency relocations by City social service agencies to better lodgings.

Out of 289 census tracts in the Bronx borough, 7 tracts lost more than 97% of their buildings to fire and abandonment between 1970 and 1980; another 44 tracts had more than 50% of their buildings meet the same fate. By the early 1980s, the South Bronx was considered one of the most blighted urban areas in the country, with a loss of 60% of the population and 40% of housing units. However, starting in the 1990s, many burned-out and run-down tenements were replaced by multi-unit housing.

Since the late 1980s, significant development has occurred in the Bronx, first stimulated by the city's "Ten-Year Housing Plan" and community members working to rebuild the social, economic and environmental infrastructure by creating affordable housing. Groups affiliated with churches in the South Bronx erected the Nehemiah Homes with about 1,000 units. The grass roots organization Nos Quedamos' endeavor known as Melrose Commons began to rebuild areas in the South Bronx. The IRT White Plains Road Line (2 and ​5 trains) began to show an increase in riders. Chains such as Marshalls, Staples, and Target opened stores in the Bronx. More bank branches opened in the Bronx as a whole (rising from 106 in 1997 to 149 in 2007), although not primarily in poor or minority neighborhoods, while the Bronx still has fewer branches per person than other boroughs.

In 1997, the Bronx was designated an All America City by the National Civic League, acknowledging its comeback from the decline of the mid-century. In 2006, The New York Times reported that "construction cranes have become the borough's new visual metaphor, replacing the window decals of the 1980s in which pictures of potted plants and drawn curtains were placed in the windows of abandoned buildings." The borough has experienced substantial new building construction since 2002. Between 2002 and June 2007, 33,687 new units of housing were built or were under way and $4.8 billion has been invested in new housing. In the first six months of 2007 alone total investment in new residential development was $965 million and 5,187 residential units were scheduled to be completed. Much of the new development is springing up in formerly vacant lots across the South Bronx.

In addition came a revitalization of the existing housing market in areas such as Hunts Point, the Lower Concourse, and the neighborhoods surrounding the Third Avenue Bridge as people buy apartments and renovate them. Several boutique and chain hotels opened in the 2010s in the South Bronx.

New developments are underway. The Bronx General Post Office on the corner of the Grand Concourse and East 149th Street is being converted into a market place, boutiques, restaurants and office space with a USPS concession. The Kingsbridge Armory, often cited as the largest armory in the world, is scheduled for redevelopment as the Kingsbridge National Ice Center.

Under consideration for future development is the construction of a platform over the New York City Subway's Concourse Yard adjacent to Lehman College. The construction would permit approximately 2,000,000 square feet (190,000 m²) of development and would cost US$350–500 million.

The Census Bureau considers the Bronx to be the most diverse area in the country. There is an 89.7 percent chance that any two residents, chosen at random, would be of different race or ethnicity. The borough's most populous racial group, white, declined from 99.3% in 1920 to 44.9% in 2018.

The Bronx contains 532,487 housing units, with a median value of $371,800, and with an owner-occupancy rate of 19.7%, the lowest of the five boroughs. There are 495,356 households, with 2.85 persons per household. 59.3% of residents speak a language besides English at home, the highest rate of the five boroughs.

In the Bronx, the population is 7.2% under 5, 17.6% 6-18, 62.4% 19–64, and 12.8% over 65. 52.9% of the population is female. 35.3% of residents are foreign born.

The per capita income is $19,721, while the median household income is $36,593, both being the lowest of the five boroughs. 27.9% of residents live below the poverty line, the highest of the five boroughs.

According to the 2010 Census, 53.5% of Bronx's population was of Hispanic, Latino, or Spanish origin (they may be of any race); 30.1% non-Hispanic Black or African American, 10.9% of the population was non-Hispanic White, 3.4% non-Hispanic Asian, 0.6% from some other race (non-Hispanic) and 1.2% of two or more races (non-Hispanic).

As of 2010, 46.29% (584,463) of Bronx residents aged five and older spoke Spanish at home, while 44.02% (555,767) spoke English, 2.48% (31,361) African languages, 0.91% (11,455) French, 0.90% (11,355) Italian, 0.87% (10,946) various Indic languages, 0.70% (8,836) other Indo-European languages, and Chinese was spoken at home by 0.50% (6,610) of the population over the age of five. In total, 55.98% (706,783) of the Bronx's population age five and older spoke a language at home other than English. A Garifuna-speaking community from Honduras and Guatemala also makes the Bronx its home.

According to the 2009 American Community Survey, White Americans of both Hispanic and non-Hispanic origin represented over one-fifth (22.9%) of the Bronx's population. However, non-Hispanic whites formed under one-eighth (12.1%) of the population, down from 34.4% in 1980. Out of all five boroughs, the Bronx has the lowest number and percentage of white residents. 320,640 whites called the Bronx home, of which 168,570 were non-Hispanic whites. The majority of the non-Hispanic European American population is of Italian and Irish descent. People of Italian descent numbered over 55,000 individuals and made up 3.9% of the population. People of Irish descent numbered over 43,500 individuals and made up 3.1% of the population. German Americans and Polish Americans made up 1.4% and 0.8% of the population respectively.

The Bronx is the only New York City borough with a Hispanic majority, many of whom are Puerto Ricans and Dominicans.At the 2009 American Community Survey, Black Americans made the second largest group in the Bronx after Hispanics and Latinos. Black people of both Hispanic and non-Hispanic origin represented over one-third (35.4%) of the Bronx's population. Black people of non-Hispanic origin made up 30.8% of the population. Over 495,200 Black people resided in the borough, of which 430,600 were non-Hispanic Black people. Over 61,000 people identified themselves as "Sub-Saharan African" in the survey, making up 4.4% of the population.^{[citation needed]}

Native Americans are a very small minority in the borough. Only some 5,560 individuals (out of the borough's 1.4 million people) are Native American, which is equal to just 0.4% of the population. In addition, roughly 2,500 people are Native Americans of non-Hispanic origin.^{[citation needed]}

In 2009, Hispanic and Latino Americans represented 52.0% of the Bronx's population. Puerto Ricans represented 23.2% of the borough's population. Over 72,500 Mexicans lived in the Bronx, and they formed 5.2% of the population. Cubans numbered over 9,640 members and formed 0.7% of the population. In addition, over 319,000 people were of various Hispanic and Latino groups, such as Dominican, Salvadoran, and so on. These groups collectively represented 22.9% of the population. At the 2010 Census, 53.5% of Bronx's population was of Hispanic, Latino, or Spanish origin (they may be of any race). Asian Americans are a small but sizable minority in the borough. Roughly 49,600 Asians make up 3.6% of the population. Roughly 13,600 Indians call the Bronx home, along with 9,800 Chinese, 6,540 Filipinos, 2,260 Vietnamese, 2,010 Koreans, and 1,100 Japanese.^{[citation needed]}

Multiracial Americans are also a sizable minority in the Bronx. People of multiracial heritage number over 41,800 individuals and represent 3.0% of the population. People of mixed Caucasian and African American heritage number over 6,850 members and form 0.5% of the population. People of mixed Caucasian and Native American heritage number over 2,450 members and form 0.2% of the population. People of mixed Caucasian and Asian heritage number over 880 members and form 0.1% of the population. People of mixed African American and Native American heritage number over 1,220 members and form 0.1% of the population.^{[citation needed]}

The Census of 1930 counted only 1.0% (12,930) of the Bronx's population as Negro (while making no distinct counts of Hispanic or Spanish-surname residents).

At the 2010 Census, there were, 1,385,108 people living in Bronx, a 3.9% increase since 2000. As of the United States Census of 2000, there were 1,332,650 people, 463,212 households, and 314,984 families residing in the borough. The population density was 31,709.3 inhabitants per square mile (12,242.2/km²). There were 490,659 housing units at an average density of 11,674.8 per square mile (4,507.4/km²). Census estimates place total population of Bronx county at 1,392,002 as of 2012.

There were 463,212 households, out of which 38.1% had children under the age of 18 living with them, 31.4% were married couples living together, 30.4% had a female householder with no husband present, and 32.0% were non-families. 27.4% of all households were made up of individuals, and 9.4% had someone living alone who was 65 years of age or older. The average household size was 2.78 and the average family size was 3.37.

The age distribution of the population in the Bronx was as follows: 29.8% under the age of 18, 10.6% from 18 to 24, 30.7% from 25 to 44, 18.8% from 45 to 64, and 10.1% 65 years of age or older. The median age was 31 years. For every 100 females, there were 87.0 males.

The 1999 median income for a household in the borough was $27,611, and the median income for a family was $30,682. Males had a median income of $31,178 versus $29,429 for females. The per capita income for the borough was $13,959. About 28.0% of families and 30.7% of the population were below the poverty line, including 41.5% of those under age 18 and 21.3% of those age 65 or over.

From 2015 Census data, the median income for a household was (in 2015 dollars) $34,299. Per capita income in past 12 months (in 2015 dollars): $18,456 with persons in poverty at 30.3%. Per the 2016 Census data, the median income for a household was $35,302. Per capita income was cited at $18,896.