From 1880 to 1920, American cities grew rapidly mainly because of changes in the economy. After the Civil War, the American economy became more industrialized, turning the United States into the world's largest and richest economy. And as more and more Americans began to work in industry rather than on the land, large numbers of people migrated from the country to the cities.

Inevitably, this process facilitated the rapid expansion of urban areas, which subsequently experienced a massive growth in population as a result.

African American migration from South to North was also a major factor in the growth of American cities during this period. Though racial prejudice was widespread across the whole country, the North didn't have the same kind of laws in place that were used to keep African Americans in a state of subjection down South.

It was largely as a means of escaping the repressive Jim Crow laws that African Americans headed north. They also made the journey to take advantage of the growing employment opportunities being made available in the industrial concerns that seemed to be springing up everywhere.

Americans reacted to these developments in many different ways. Some formed themselves into labor unions to seek better wages and conditions.

Although the American economy expanded rapidly during this period, generating greater wealth than ever before, that wealth was by no means spread equally. While a small handful of industrialists got rich, most of their workers toiled away in unsafe conditions for little pay. In order to remedy this situation, workers formed themselves into labor unions, where they would be stronger and better able to speak to their employers with a united voice.

In 1790, only 5% of Americans lived in cities. Not even a fully century later, in 1880, that percentage had grown to 35%. By 1920, over half of Americans were city dwellers. This increase in urbanization was influenced by several factors. First and foremost, America was becoming an industrialized nation, a nation of factories and production. What's more, factories became more mechanized than ever as they started to incorporate innovations like assembly lines and interchangeable parts.

Factories, however, needed workers, and thousands of Americans needed jobs. The Civil War displaced many people, especially in the South, and they sought work in cities. Immigrants also settled in urban, industrial areas as they fled the famines and political upheavals in their own countries (especially in southern, central, and eastern Europe) and searched for their own version of the American dream. In fact, from 1870 to 1920, about 25 million immigrants arrived in the United States, and many, if not most, of them settled in cities.

Of course, the influx of population into American cities and into factory life created many challenges. Thousands of people packed into small areas, creating horrible living conditions, complete with overcrowding, poor sanitation, pollution, lack of proper food, and the spread of disease. Factories themselves were largely unsafe, and workers had few, if any, rights. They lived with constant danger, long hours, low wages, and abuse from supervisors.

All of these challenges led to social movements that tried to find solutions to urban problems and support city dwellers. The labor movement grew rapidly to address workers' struggles and push for shorter hours, higher wages, and better working conditions. People like Jane Addams sought to offer aid and improve living conditions for families packed into tenement houses. Political organizers sought changes in laws to address the challenges of city and work life, as well as the corruption of urban political machines. Indeed, the Progressive movement for social change grew up and flourished because of American urbanization, and its efforts, in time, led to improvement in the lives of at least some urban Americans.

1880–1920, what historians typically refer to as the American Gilded Age, was a period that saw massive movements of people across the country. The tremendous influx of people into the burgeoning cities of the North led in large part to their growth and the general urbanization of late-nineteenth century America. Many of the people who moved into Northern cities were recently emancipated slaves. The end of the Civil War caused a massive displacement of Southern black people, who (either because wartime service had moved them to different parts of the country or because they were no longer tied to their masters’ estates) migrated in enormous numbers to the North. This event is known as the Great Migration in American history, and it was during this time that the population of Northern cities like Chicago, New York, Pittsburgh, and Cincinnati exploded.

In addition, the Civil War itself precipitated fundamental change in the economy of the Northern cities. The unified war effort saw the standardization of money, the Union government borrow heavily from municipal banks (which increased the power of these banks), the accelerated construction of railroads, the centralization of industry, and many other events of this nature. The result of the cumulative concentration of Northern productive capacity was an expansion in both size and importance of major urban centers. The need to feed and clothe Union soldiers during the war, for example, led to the establishment of enormous feedlots, slaughterhouses, and textile factories in cities across the north, which became mainstay establishments in the period after emancipation.

Finally, once the central institutions of the Northern economy had been created, the new wealth that business made in Northern cities attracted a vast array of foreign immigrants, many of whom dreamed of making a new life in America. For example, New York City became one of the most ethnically and nationally diverse places in the world. The massive inflow of immigrants, primarily from Italy, Germany, and Eastern Europe, similarly added to the growth of urban America.

The history of large-scale urbanization is closely tied to the history of industrialization and the rise of the industrial economy. Note the degree to which industrialization actually requires large concentrations of labor to effectively function. As economies grow more industrialized, cities likewise grow larger and more numerous. Note also that these demographic trends would have only been further magnified by the extraordinarily high rates of immigration that can be observed in the decades surrounding the turn of the century, further bolstering this history of urban growth.

The effects of this transformation were far-reaching and profound. For one thing, growing population size and density posed various challenges that required solutions. The proliferation of crime along with the widespread threat of fires led to the rise of professional police and firefighting forces. Additionally, you can point towards the rise of mass transportation as an additional innovation aimed at meeting the needs of an increasingly urbanized society. On the other hand, it is also worth remembering the history of tenements and the miserable living conditions faced by the lower classes and the urban poor. In some respects, these urban populations innovated in response to the new challenges posed by increasingly urban societies, and in others they either failed to do so (or did so insufficiently).

At the same time, it is also worth recognizing the ways in which urbanization was closely tied with the history of the political machines. Machine politics ran on a quid-pro-quo basis, drawing support from immigrants, the lower classes, and the urban poor. In essence, the machines promised a safety net that otherwise did not yet exist in the United States, which they then used to mobilize large voting blocs in support of chosen candidates, allowing the machines to accrue tremendous political power for themselves. Remember, however, that the political machines carried with them a reputation for political corruption (a tendency most famously exemplified in the example of Boss Tweed of Tammany Hall), even as they continued to represent a fundamental political reality within this history of urban growth.